(A classical definition)
Although, many would be knowing this, for some this information can be new, for some this will mean precise description of some general terms. This article has become "technical", but I hope it has not become boring. And I also hope, this is helpful to the Ghazal fans.
Instead of giving my personal views, I thought of quoting somebody who is an authority. There is book/dictionary/collection of She'rs titled "Aaina-e-Ghazal", which is a treasure for every Ghazal fan.There goes a long essay - "Ghazal kya hai ?" by Dr. Arshad Jamaal.
The essay is written in Hindi, and is about History of Ghazal, its development, its milestones, important Shayars etc. One part of it describes the definition of Ghazal. The following is loosely based on that. The essay talks only about what is a Ghazal. To that I have added in the following, what is not a Ghazal. So any mistakes in these parts, are mine. [ These are enclosed in square brackets like this. ]
Also one thing should be kept in mind that, this is not mathematics. So "preciseness" of the "definition" should not be questioned.
Ghazal in short, is a collection of She'rs which follow the rules of 'Matla', 'Maqta', 'Beher', 'Qaafiyaa' and 'Radif'. So in order to know what Ghazal is, it's necessary to know what these terms mean.
To understand these terms easily , we will take an example.
1. koi ummeed bar nahin aati
koi surat nazar nahin aati
2. aage aati thi haal-e- dil pe hansi
ab kisi baat par nahin aati
3. hum wahan hain, jahan se humko bhi
kuch hamaari khabar nahin aati
4. kaabaa kis munh se jaaoge 'Ghalib'
sharam tumko magar nahin aati
What is a She'r ?
It is a poem of two lines. This definition is
deceptively simple. Please note that, every She'r is a poem in itself ! A She'r
does not need, anything around it, to convey the message.
All the 4 stanzas in our example are independent poems, Sher's.
So Ghazal is necessarily a collection of two-line-poems
[ Rafi's "rang aur noor ki baaraat kise pesh karun" is NOT a Ghazal, as every stanza is of 3 lines, and not of 2. ]
What are other restrictions ? Many, and important
[ Any collection of She'rs is not Ghazal. Some good examples are ; the famous Mukesh song from Yahoodi, "yeh mera deewaanaapan hai" ; and the title song of "dil apana aur preet parayi". Each stanza in these songs can be considered as an independent She'r, but they are NOT Ghazals. To understand, why, we have to wait till 'Qaafiyaa, 'Radif'. ]
What is 'Beher' ?
'Beher' is the 'meter' of the She'rs. It can be considered as the length of the She'r. Both the lines in the She'r *MUST* be of same 'Beher'. And all the She'rs in one Ghazal *MUST* be of the same 'Beher'. There are 19 (!!) kinds of 'Beher'. But in simple terms, 'Beher' is categorized in 3 classes : Short, medium and long.
The examples in  and italicised are my additions, from Hindi Films.
ahale dairo-haram reh gaye
tere deewane kam reh gaye
[ Also Talat's "dil-e-nadan tujhe hua kya hai" ]
umr jalwon men basar ho, yeh zaroori to nahin
har shab-e-gam ki seher ho, yeh zaroori to nahin
[ And by Gulzar, "ruke ruke se kadam, ruk ke baar baar chale" ]
ai mere humnashin, chal kahin aur chal, is chaman men ab
apnaa guzaaraa nahin
baat hoti gulon ki, to seh lete hum, ab to kaaton pe bhi haq hamaaraa nahin
[ The filmfare winner, "Manzilen apani jagah hain" !! Yes ! It IS a Ghazal. And the Shayar is Prakash Mehra !]
So Ghazal is a collection of She'rs of the SAME 'Beher'.
What is 'Radif' ?
In a Ghazal,the second line *MUST* end with the *SAME* word/s. This repeating of common words is the 'Radif' of the Ghazal.
In our example, the 'Radif' is "nahin aati".
[ Sometimes, the Ghazal becomes known by its 'Radif'. eg. "jaraa aahista chal" sung by Pankaj Udhas. We all know one Ghazal by it's 'Radif' as "aahista aahista"]
What is 'Qaafiyaa' ?
'Qaafiyaa' is the rhyming pattern which all the words before 'Radif' *MUST* have.
In our example the 'Qaafiyaa' is "bar", "nazar", "par", "magar" etc. This is a necessary requirement. Something which is followed even in the exceptions to all these rules.
So Ghazal is a collection of She'rs of
ending in same 'Radif' and having same 'Qaafiyaa'.
[ That's the reason, why "yeh mera diwanapan hai" etc. are NOT Ghazals. There is no common thing which can be called 'Qaafiyaa' and 'Radif'. ]
What is 'Matla' ?
The first She'r in the Ghazal *MUST* have 'Radif' in its
both lines. This She'r is called 'Matla' of the Ghazal and the Ghazal is usually
known after its 'Matla'. There can be more than one 'Matla' in a Ghazal. In such
a case the second one is called 'Matla-e-saani' or 'Husn-e-matla'.
In our example, the first She'r is the 'Matla'.
What is 'Maqta' ?
A Shayar usually has an alias ie. 'takhallus' e.g. Mirza Asadullah Khan used 'Ghalib' as his 'takhallus' and is known by that. Other examples are 'Daag' Dehlvi, 'Mir' Taqi Mir, Said 'Rahi', Ahmed 'Faraz' etc. There is a She'r in a Ghazal, the last one, which has the Shayar's 'takhallus' in it.
[ A Shayar, can use the 'Maqta' very intelligently. He can "talk to himself" like one in our example. I have lots of favourite She'rs which are 'Maqta' of some Ghazal. Some gems are:
koi nam-o-nishan puchhe to ai qaasid bataa denaa,
takhallus 'Daag' hai, aur aashiqon ke dil men rehte hain
jab bhi milte hain, to kehte hain, "kaise ho 'Shakeel'",
iske aage to koi baat nahin hoti hai
The first one uses the meaning of the 'takhallus' to create the magic, and the second one is just simple, simply beautiful. ]
To summarize, Ghazal is a collection of She'rs (independent two-line poems), in which there is atleast one 'Matla', one 'Maqta' and all the She'rs are of same 'Beher' and have the same 'Qaafiyaa' and 'Radif'.
Exceptions And Important Points to Note
1. Ghazal is just a form. It is independent of any language. e.g. in Panjabi also, there can be (and there are) good Ghazals.
2. Some Ghazals do NOT have any 'Radif'. Rarely. Such Ghazal's are called "gair-muraddaf" Ghazal.
3. Although, every She'r, should be an independent poem in itself, it is possible, that all the Sher's are on the same theme. What famous example can be other than "chupke chupke raat din aansoo bahaanaa yaad hai".
4. In modern Urdu poetry, there are lots of Ghazals which do NOT follow the restriction of same 'Beher' on both the lines of She'r. [ My example in 'Maqta', the She'r by Shakeel, is one. ] But even in these Ghazals, 'Qaafiyaa' and 'Radif' are present.
5. The restriction of 'Maqta' is really very loose. Many many Ghazals do NOT have any 'Maqta'. [ I think 'Maqta' was used in the earlier times, as a way to keep the credit. But since this is traditional, many Ghazals do have a 'Maqta' just for the sake of it. Sometimes the name of the Shayar comes unnaturally in the last She'r of the Ghazal.
I would like to end this essay of mine quoting that beautiful definition from Jagdish Bhatnagar 'Hayat' :
''Ghazal rudaad hai naakaamiyon ki,
Ghazal mehrumiyon ki daastaan hai |
Ghazal riste hue zakhmon ka marham,
Ghazal ik chaaraa-e-dard-e-nihan hai |
Ghazal ka husn hi hai, husn-e-aalam,
Ghazal ka noor hi noor-e-jahan hai ''