| Definition -
(Aka formal agreement)
The agreement of the parts of a sentence (e.g. number, person, and gender) that is required by the conventional rules of grammar.
For example, in English we don't say I is, because according to the conventional rules of English grammar is cannot be used when the subject is I. Instead, we say I am and that is what "sounds" correct to almost all English speakers.
This can be constrasted with notional concord, which is concord based on the meaning of the words. For example, in the sentence The committee have decided to eat lunch a plural verb is "incorrectly" used with a singular collective noun committee because the meaning of the words makes this "sound" correct to us.