The term fasa:hah describes: 1) Word, 2) Discourse, and 3) Speaker.
1) The Fasa:hah of The Word:
For any word to be considered fasi:h (adjective from fasa:hah) four conditions must be met:
[1] The sounds (phonemes) that form the word must be in harmony with one another: so that it is not difficult to pronounce as a result of ‘disagreement’ between the places of articulation of each sound (letter), for example. This does not mean that the cause of the difficulty is the nearness of the places of articulation as we find that sounds that form words like al-jaysh ‘الْجَيْش: the army’, al-famm ‘الْفَمّ: the mouth’, al-shajar ‘الشَّجَر: the trees’, etc., are harmonious while a word like mala‘ ‘مَلَع: to hurry’ whose sounds’ places of articulation are not so near does not sound as good or as easy to pronounce. 
Also the length of the word is not one of the main things that decide whether it is fasi:h or not. Long words like sahsalaq ‘صَهْصَلَق: strong loud sound’ and khanshali:l ‘خَنْشَلِيل: sword’ are not fasi:h, unlike the fasi:h words of layastakhlifannahum ‘{لَيَسْتَخْلِفَنَّهُمْ}: he will, of a surety, grant them succession to’, and fasayakfi:kahum ‘{فَسَيَكْفِيكَهُمُ}: He will suffice you against them’, which are even longer. The judge then is nothing but ‘good taste’ that can distinguish the good and bad sounds.
[2] The word in most cases has to be familiar and clear in meaning. Therefore, a word is not considered fasi:h in case:
a) it proves difficult to know what it means and one has to look hard for its meaning. While words such as mushanfirah ‘مُسْحَنْفِرَة: wide ’, bu‘a:q ‘بُعَاق: rain‑cloud’, jardahal ‘جَرْدَحَل: valley’ and juhaysh ‘جُحَيْش: tyrant or dictator’ can be found in some dictionaries, jahlanja‘ ‘جَحْلَنْجَع’ is not listed.
b) if a meaning has to be coined for it as it might be far fetched. For example, linguists differed about musarrajan ‘مُسَرَّجًا’ in the following lines:

أَيَّامَ أَبْدَتْ وَاضِحًا مُفَلَّجَا

أَغَرَّ بَرَّاقًا وَطَرْفًا أَبْرَجَا

وَمُقْلَةً وَحَاجِبًا مُزَجَّجَا

وَفَاحِمًا وَمَرْسِنًا مُسَرَّجَا
 
ascribed to Ru’bah ibn Al-‘Aja:j, as they could not understand what is meant by describing his beloved’s nose as musarrajan. The meaning is ambiguous.
[3] it is in conformity with the rules of the language and used in its regular form. Al-Farazdaq says:

خُضُعَ الرِّقَابِ نَوَاكِسَ الأَبْصَارِ

وَإِذَا الرِّجَالُ رَأَوْا يَزِيدَ رَأَيْتَهُمْ
 
In this example the underlined nawa:kis ‘looking down’ which is feminine plural is used to refer to al-rija:l ‘الرِّجَالُ: men’ which is obviously masculine. Therefore, the word nawa:kis is not considered fasi:h in this context because it breaks the rule. The correct form in this context is the masculine na:kisi: ‘نَاكِسى’.
Abi Al-Najm ibn Quda:mah says:

أَنْتَ مَلِيكُ النَّاسِ رَبًّا فَاقْبَلِ

الْحَمْدُ لِلَّهِ الْعَلِىِّ الأَجْلَلِ
 
In this line the correct form of al-’Ajlal ‘الأَجْلَلِ: Owner of Majesty’ is al-’Ajal ‘الأَجَلِّ’ with one la:m and a shaddah, ‘لِّ’, not two la:ms, ‘لَلِ’, as the line says which also changed the way the ji:m ‘ج’ is pronounced.
Jami:l also used hamzat al-qat‘ ‘هَمْزَة الْقَطْعِ’ where he should have used hamzat al-wasl ‘هَمْزَة الْوَصْلِ’ in the word ithnayn ‘اثْنَيْن: two’. He says: 

“عَلَى حَدَثَانِ الدَّهْرِ مِنِّى وَمِنْ جُمْلِ

أَلا لا أَرَى إِثْنَيْنِ أَحْسَنَ شِيمَةً”
 
An exception to this above mentioned rule are the words that the Arabs used in their irregular forms preferring them to their regular forms because they are pronounced with more ease and also sounded better, such as istahwadha ‘اسْتَحْوَذَ: to have control over something’ instead of istaha:dha ‘اسْتَحَاذَ’, qatata ‘قَطَطَ: to cut (one’s hair)’ instead of qatta ‘قَطَّ’, etc.
[4] The way the word as a whole sounds should be acceptable to the ‘ear’ (nice to hear) and agree with good taste.
A poet said: “وَأَحْمَقٌ مِمَّنْ يَلْعَقُ الْمَاءَ قَالَ لِى دَعِ الْخَمْرَ وَاشْرَبْ مِنْ نُقَاخٍ مُبَرَّدِ”
 
(A fool from among those who lick the water said to me:

“Do not drink alcohol, and drink cold water instead”.)
[i]Here, the word nuqa:kh ‘نُقَاخٍ: sweet water’ is very heavy on the ear, and the listener would realise immediately that the word is out of place. The same is correct when describing a good-looking person as ‘utbu:l ‘عُطْبُول’. 

2) The Fasa:hah of The Discourse:
For any discourse to be described as fasi:h, it has to be clear in meaning, easily uttered, well-structured, free of ambiguity and conforming to the rules of the language. For this to be achieved, the discourse has to meet four conditions:
[1] The words that are put together to form a discourse must be harmonious with one another in a way that makes pronunciation of phrases/sentences easy.
The lack of harmony and difficulty in pronunciation may be sometimes due to one or more of the following points:
a) Repetition of the same letters:
Al-Ja:hiz recited: “وَقَبْرُ حَرْبٍ بِمَكَانِ قَفْرٍ وَلَيْسَ قُرْبَ قَبْرِ حَرْبٍ قَبْرُ”.
 Here, the repetition of the letters qa:f ‘ق’, ra:’ ‘ر’ and ba:’ ‘ب’ in the way they appear in this line render the pronunciation of the line as a whole extremely difficult, although taken individually, each word may not show such difficulty.
Al-Hari:ri also says: “وَعَافَ عَافِى الْعُرْفِ عِرْفَانَهُ”. The repetition of the ‘ayn ‘ع’ and the fa:’ ‘ف’ causes a similar problem.
In contrast to these examples, in Q84:8 we have {فَسَوْفَ يُحَاسَبُ حِسَابًا يَّسِيرًا}, Q68:9 reads {وَدُّوا لَوْ تُدْهِنُ فَيُدْهِنُونَ}and Q11:48 has the sound /m/ repeated eight times successively: {أُمَمٍ مِّمَّن مَّعَكَ}.
 As is clear from these examples, in spite of the difficulty arising from the feature of repeating the same sounds in the above lines of poetry, this is not encounterd in the Qur’an!
b) Verbs following one another, whether belonging to different tenses, such as Al-Qa:di Al-’Arraja:ni’s line of verse: 

“بِالنَّارِ فَرَّقَتِ الْحَوَادِثُ بَيْنَنَا وَبِهَا نَذَرْتُ أَعُودُ أَقْتُلُ رُوحِى”

(Lit., Fire has caused us to part, and by it I swear I return I kill myself),
or to the same tense as in Al-Mutanabbi’s ambiguous line: 

“أَقِلْ أَنِلْ أَقْطِعْ احْمِلْ عَلِّ سَلِّ أَعِدْ زِدْ هَشَّ بَشَّ تَفَضَّلْ أَدْنِ سُرَّ صِلِ ”

(Lit., help, give, give (land), give (horse), raise rank, remove (the cause of my concern), get me back (to my previous place of honour), 

give me more (of your bounty), show happiness, smile, give (honour), bring me near (your grace), please, give (always)),
cause the same sort of difficulty like the use of many adjectives following one another as in Al-Mutanabbi’s: 

“دَانٍ بَعِيدٍ مُحِبٍّ مُبْغِضٍ بَهِجٍ أَغَرَّ حُلْوٍ مُمِرٍّ لَيِّنٍ شَرِسِ

نَدٍ أَبِىٍّ غَرٍ وَافٍ أَخِى ثِقَةٍ جَعْدٍ سَرِىٍّ نَهٍ نَدْبٍ رِضًى نَدِسِ” 

(near, far, loving, hateful, rejoicing (happy),

noble in action, sweet, bitter, kind, wild,

giving, proud, do-gooder, loyal, trustworthy,

generous, of noble birth, intelligent, swift in responding to helping others, accepting, knowing.)
A huge difference appears when contrasting these many adjectives to Q68:10-13 {وَلا تُطِعْ كُلَّ حَلافٍ مَّهِينٍ * هَمَّازٍ مَّشَّآءٍ بِنَمِيمٍ * مَّنَّاعٍ لِّلْخَيْرِ مُعْتَدٍ أَثِيمٍ * عُتُلِّ بَعْدَ ذَلِكَ زَنِيمٍ} (Khan&Helali: “And obey not everyone who swears much, - and is considered worthless, a slanderer, going about with calumnies, hinderer of the good, transgressor, sinful, cruel, - after all that base-born (of illigitimate birth)”),
where the successive Arabic adjectives read much more smoothly and do not sound unnatural like the above two lines. Another example is Q66:5.
 
[2] Weak composition due to breaking the Arabic grammatical rules render the discourse non-fasi:h.
Joining the two pronouns together where the latter should have been placed before the former, as in Al-Mutanabbi’s: 

“خَلَتِ الْبِلادُ مِنَ الْغَزَالَةِ لَيْلَهَا فَأَعَاضَهَاكَ اللَّهُ كَىْ لا تَحْزَنَا” 

(When the sun disappears in the evening in the land,

Allah makes it up to the people through you so that they may not be upset over its setting).
is a good example as fa’a‘a:daha:ka ‘فَأَعَاضَهَاكَ’ should have been fa’a‘a:dakaha ‘فَأَعَاضَكَهَا’ or fa’a‘a:daha: ’iyya:ka ‘فَأَعَاضَهَا إِيَّاكَ’. Similarly is Hassa:n ibn Tha:bit’s:

“وَلَوْ أَنَّ مَجْدًا أَخْلَدَ الدَّهْرَ وَاحِدًا مِنَ النَّاسِ أَبْقَى مَجْدُهُ الدَّهْرَ مُطْعِمَا” [CENTER][/CENTER]

(Lit., If Glory was to cause only one (man) from among the people to live for ever,

Mut‘im’s glory would have caused him to live eternally).
While the pronoun in majduhu ‘مجْدُهُ’ (his glory) refers to Mut‘im ‘مُطْعِمَا’, the two words are so distant that they render the line non-fasi:h.
 
[3] Structural complexity, i.e., putting the words in the wrong order or using an ambiguous word, etc., affect the fasa:hah of the discourse.
Al-Farazdaq says: إِلَى مَلِكٍ مَا أُمُّهُ مِنْ مُحَارِبٍ أَبُوهُ وَلا كَانَتْ كُلَيْبٌ تُصَاهِرُهُ,

(Lit., To a king whose mother is not from Muha:rib,

his father (is), nor was Kulayb in a marriage relationship with him)
instead of “أَبُوهُ وَلَيْسَتْ أُمُّهُ مِنْ مُحَارِبٍ” (Lit., his father, and not his mother, is from Muha:rib). This leads to ambiguity and clumsiness of style. He also does the same in:

“وَمَا مِثْلُهُ فِى النَّاسِ إِلا مُمَلَّكًا أَبُو أُمِّهِ حَىٌّ أَبُوهُ يُقَارِبُهُ” 

(Lit., there is no one like him among the people, except Mumallak,

his mother’s father, alive, his father, who is like him (in honour)).
Another poet said: فَأَصْبَحَتْ بَعْدَ خَطَّ بَهْجَتِهَا كَأَنَّ قَفْرًا رُسُومَهَا قَلَمَا,
 
that is, (It became (fell) after, drew, being a happy place as if, (into) ruins, its lines, a pen) instead of “فَأَصْبَحَتْ بَعْدَ بَهْجَتِهَا قَفْرًا كَأَنَّ قَلَمَاخَطَّ رُسُومَهَا” (After being a happy place, it fell into ruins as if a pen had drawn its lines broken and twisted).
 
[4] Meaning complexity in the sense that the words used are not the right ones to convey the meaning intended especially ambiguous and/or far fetched metaphors.
[i]The use of ’alsun ‘أَلْسُن’ (tongues) instead ’a‘yun ‘أَعْيُن’ (eyes) in ‘أَرْسَلَ الْحَاكِمُ أَلْسِنَتَهُ فِى الْمَدِينَةِ’ (The ruler has sent his ’alsun in the town) to mean ‘spies’ makes the utterance ambiguous and therefore non-fasi:h. The same applies if a person uses expressions like ‘بَيْتُهُ كَثِيرُ الْجِرْذَانِ’ (His house is full of rats) or ‘أَبْيَضٌ سِرْبَالُ الطَّبَّاخِ’ (Clean-white is the chef’s uniform) to refer to dirt and cleanliness respectively. This would be considered complicated, if not wrong, as in Arabic these expressions refer to ‘plenty of food’ for the former, and ‘miserliness’ in the latter. 

3) The Fasa:hah of The Speaker:
A speaker is considered fasi:h when he is able to express himself properly in different situations and for different puposes using fasi:h discourse; it is therefore a personal quality.
[i] Mu‘a:wiyah once asked some people in his presence about who the most fasi:h of the Arabs were. The answer he got was: “Those whose speech is free from the lakhlakha:niyyah ‘لَخْلَخَانِيَّة’ (common feature of a dialect) of Iraq, the ‘an‘anah ‘عَنْعَنَة’ of Tami:m, the kaskasah ‘كَسْكَسَة’ of Bakr, the ghamghamah ‘غَمْغَمَة’ of Quda:‘ah and the tumtuma:niyyah ‘طُمْطُمَانِيَّة’ of Himyar. He asked: “Who are they?”. “Quraysh”, was the answer. 

The Concept of Al-Bala:ghah
The word bala:ghah ‘بَلاغَة’ is derived from balagha ‘بَلَغَ’ (to reach, arrive at, get to). Balgha al-gha:yah ‘بَلَغَ الْغَايَةَ’ means ‘to reach the end’, that is, to achieve one’s goal. A person described as bali:gh ‘بَلِيغ’ (adjective from bala:ghah) is one who says/has the ability to say what should be said using fasi:h words/discourse in conformity with what the given situation requires.

Therefore, what is said when a person is sad, happy, excited, preaching, talking to an adult, a child, a soldier, a thief, etc., differ in many ways according to the situation. The fundamental point in bala:ghah is expressed in the statement of Al-Hutay’ah to ‘Umar ibn Al-Khatta:b: “تَحَنَّنْ عَلَىَّ هَدَاكَ الْمَلِيكُ فَإِنَّ لِكُلِّ مَقَامٍ مَّقَالا”, [CENTER][/CENTER]
(Treat me with mercy, may the King (Allah) guide you,



as for every situation there is a particular utterance to suit it).
It is the second half of this line that pinpoints the core of bala:ghah, as it states that every context requires its own distinctive form of speech. Apparently, this is the main reason that there is a consensus that bala:ghah is defined as:
“مُطَابَقَةُ الْكَلامِ لِمُقْتَضَى الْحَالِ (مَعَ فَصَاحَتِهِ)”,
that is, the conformity of the utterance to the requirements of the situation using fasi:h discourse.
 
In a poem praising Hisha:m ibn ‘Abd Al-Malik, Abu Al-Najm said: 

“صَفْرَاءُ قَدْ كَادَتْ وَلَمَّا تَفْعَلِ كَأَنَّهَا فِى الأُفْقِ عَيْنُ الأَحْوَلِ”, 

that is, (the sun), looking yellow as it was about to set, 

looked on the horizon like the eye of a cross-eyed person).
As Hisha:m was cross-eyed himself, he ordered the poet to be put in prison. Here, the utterance did not conform to what the situation required. So, although the words, discourse and speaker may be described as fasi:h, the speech cannot be described as bali:gh.
 
In contrast with this, when inviting the king of Persia to Islam, the Prophet Muhammad (r), knowing that he was addressing a non-Arabic speaking person, wrote to him in a style that is direct, clear, easy to translate and understand. He (r) wrote: 
"مِنْ مُحَمَّدٍ رَسُولِ اللَّهِ إِلَى كِسْرَى أَبْرَوَيْزَ عَظِيمِ فَارِسَ سَلامٌ عَلَى مَنِ اتَّبَعَ الْهُدَى وَآمَنَ بِاللهِ وَرَسُولِهِ فَأَدْعُوكَ بِدَاعِيَةِ الإِسْلامِ فَإِنِّى رَسُولُ اللهِ إِلَى الْخَلْقِ كَافَّةً؛ لِيُنذِرَ مَنْ كَانَ حَيًّا وَيَحِقَّ الْقَوْلُ عَلَى الْكَافِرِينَ، فَأَسْلِمْ تَسْلَمْ، فَإِنْ أَبَيْتَ فَإِثْمُ الْمَجُوسِ عَلَيْكَ".
 
On the other hand when the Prophet (r) wrote to Wa:’il ibn Hujr Al‑Hadrami and his people, he (r) used a completely different style that agreed with the linguistic abilities and nature of his Arab addressees. The message went: 
"مِنْ مُحَمَّدٍ رَسُولِ اللَّهِ إِلَى الأَقْيَالِ الْعَبَاهِلَةِ، وَالأَرْوَاعِ الْمَشَابِيبِ مِنْ أَهْلِ حَضْرَمَوْتَ بِإِقَامَةِ الصَّلاةِ وَإِيتَاءِ الزَّكَاةِ، عَلَى التَّيْعَةِ الشَّاةُ لامُقَوَّرَةُ الأَلْيَاطِ، وَلا ضَنَاكٌ، وأَنْطُوا الثَّبْجَةَ، وَالتَّيْمَةُ لِصَاحِبِهَا، وَفِى السُّيُوبِ الْخُمْسُ، لا خِلاطَ وَلا وِرَاطَ، وَلا شِنَاقَ وَلا شِغَارَ، وَمَنْ زَنَى مِمْ بِكْرٍ فَاصْقَعُوهُ مِائَةً، وَاسْتَوْفِضُوهُ عَامًا، وَمَنْ زَنَى مِمْ ثَيِّبٍ فَضَرِّجُوهُ بِالأَضَامِيمَ. وَلا تَوْصِيمَ فِى الدِّينِ، وَلا غُمَّةَ فِى فَرَائِضِ اللهِ تَعَالَى، وَمَنْ أَجْبَى فَقَدْ أَرْبَى، وَكُلُّ مُسْكِرٍ حَرَامٌ، وَائِلُ بْنُ حُجْرٍ يَتَرَفَّلُ عَلَى الأَقْيَالِ".
 
[i]It is therefore clear that bala:ghah describes both the discourse and the speaker* but not individual words.* However, bala:ghah requires extreme care in selecting the words and style that would be used in a given situation. The entire Qur’an is the matchless example of both fasa:hah and bala:ghah.