What does a. stand for? Is it adjective?
This has also been discussed recently in the thread Adjective or not?
It makes good reading.
Hi. Formally, it's an indefinite article (a man = any man). Compare to "the", which is a definite article: the man on the corner (no other man). Now, it can be considered a preposition only in informal speech when replacing "of": sorta sad (sort of sad) or a kinda mess (a kind of mess).

(By the way, I'm new to the Forum)
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 Mike in Japan's reply was promoted to an answer.
I think our Guest merely meant to ask what "a." stands for when it's an abbreviation of something, to which I must answer that it depends on the context. Where did you find it?
"a" is an articile.
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Is 'A' an adjective?
Is 'a' or 'an' used as adjective.