Conventional
late 15th century (in the sense ‘relating to a formal agreement or convention’): from French conventionnel or late Latin conventionalis, from Latin conventio(n- ) ‘meeting, covenant’, from the verb convenire (see convene).
wiktionary
convention + -al
etymonline
conventional (adj.)
late 15c., "of the nature of an agreement," from Late Latin conventionalis "pertaining to convention or agreement," from Latin conventionem (nominative conventio) "a meeting, assembly; 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").
Meaning "of the nature of a formal meeting of delegates" is from 1812, now rare; that of "established by social convention, arising out of custom or usage" is from 1761; sense of "following tradition" is from 1831. Of weapons, "non-nuclear," from 1955. Related: Conventionally.
