|Definition - When a verb (incorrectly) agrees with the nearest noun, rather than with the sentence's more distant subject.|
(1) One of your friends are here.
(This is an example of proximity concord because the verb is — incorrectly — agreeing with the noun friends, not with the singular subject one.)
(2) One of your friends is here.
(This is an example of grammatical concord because the verb is in proper agreement with the subject one.)