Usage of articles: A definition of the/a/(none) chair.

[Twin]

I am wondering which of the following phrases is correct:

a definition of a chair
a definition of the chair
a definition of chair

Perhaps they are all correct. Does the answer change if the word definition is replaced by concept?

Could you lend me a helping hand, please?
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Feel free to correct to correct my English. Any comment is welcome.

[Haihao]

Interesting! I am not very sure but I think,

1. a definition of a chair = a definition of a thing so called 'chair'
2. a definition of the chair = a definition of the specific chair refered by the speaker.
3. a definition of chair = a definition of the thing called 'chair'

My two cents.

[Twin]

Can "a definition of the chair" refer not to a specific chair (the one that has just been mentioned) but to the whole category of chairs similarly to the way in which the computer in "The computer has revolutionized the way we're living" refers to the whole category of computers and not to a specific one?
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Feel free to correct to correct my English. Any comment is welcome.

[Haihao]

I think it can, of course. Everything depends on context, I think. Maybe irrelevant and you may know it well enough that in a certain context, such expressions as 'I is', 'I says', 'A I ...' etc. are all possible.