It can be either of those things (or neither of them), depending on what other word you're talking about.
Both are correct.
Synonym means: a word having the same or nearly the same meaning as another in the language. Definition means: the act of defining or making definite, distinct, or clear / the formal statement of the...
The word no can be replaced in some contexts by none, negative, zero, or nothing.
He is a pronoun. The only synonym it has is to use the name of the person it refers to, i.e., "He is short." "John is short."