There are two different meanings of "complement" on the Cambridge Dictionary.
to make something else seem better or more attractive when combining with it
a part of a clause that usually follows the verb in English and adds more information about the subject or object
with the latter, the dictionary tags it as "LANGUAGE"
I guess the tag means the word "complement" serves as a English grammar term with that meaning.
By "term", I mean
a word or expression used in relation to a particular subject, often to describe something official or technical
In this context, the "particular subject" is English grammar.
In contrast, the first meaning does not serves as a term. Could I call it "plain English"?
Is my understanding correct?
If that is what it means, why the dictionary doesn't give "synonym" that tag? If "synonym" is just plain English, what is the corresponding English grammar term?