No. an adjective is used to describe something, just like what u said: “big” dog. a synonym is a word that has exactly or nearly the same meaning of another word. for example: pretty & beautiful, tiny & small. hope this helped :)
A synonym is something that has a similar meaning and replaces a word. For example: the word “Big” You could say: large, gigantic, humongous, etc. What you are describing is an adjective, somethings that describes a noun.
