early 15c., convencioun, "a formal agreement, covenant, treaty," also "a formal meeting or convention" (of rulers, etc.), also "a private or secret agreement," from Old French convencion "agreement" and directly from Latin conventionem (nominative conventio) "a meeting, assembly; an agreement," noun of action from past-participle stem of convenire "unite, be suitable, agree, assemble," from assimilated form of com "with, together" (see con-) + venire "to come" (from PIE root *gwa- "to go, come").
Originally of princes, powers, and potentates. In diplomacy, of agreements between states, from mid-15c.; of agreements between opposing military commanders from 1780. Meaning "a formal or recognized 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," it is attested by 1747 (in a bad sense, implying artificial behavior and repression of natural conduct, by 1847). Hence "rule or practice based on general conduct" (1790).