convention (n.) Look up convention at
early 15c., convencioun, "a formal agreement, covenant, treaty," also "a formal meeting or convention" (of rulers, etc.), also "a private or secret agreement," from Middle French convention and directly from Latin conventionem (nominative conventio) "meeting, assembly, covenant," noun of action from past participle stem of convenire "unite, be suitable, agree, assemble," from com- "together" (see com-) + venire "to come" (see venue).

Originally of princes, powers, and potentates. In diplomacy, of agreements between states, from mid-15c.; of agreements between opposing military commanders from 1780. Meaning "assembly of persons for a common objective," especially involving legislation or deliberation is from mid-16c. Conventions were important in U.S. history and the word is attested in colonial writings from 1720s; in reference to political party nomination meetings by 1817 (originally at the state level; national conventions began to be held in the 1830s).

In the social sense, "general agreement on customs, etc., as embodied in accepted standards or usages" (sometimes in a bad sense) by 1747. Hence "rule or practice based on general conduct" (1790).