|Definition - When words have the same number, gender, case, or person.|
(1) In We are a family the subject We and the verb are in agreement.
(2) In We is family they are not in agreement.
Oxford English Dictionary - The term's first citation is from 1669:
"The agreement of words together in number, gender, case, and person, which is called concord."
(Milton Gram. ii. Concords (1847) 468/1)