1580s, "that which is added," also "act of acceding" (by assent, to an agreement, etc.), from Latin accessionem (nominative accessio) "a going to, approach; a joining; increase, enlargement," noun of action from past-participle stem of accedere "approach, enter upon" (see accede). From 1640s as "act of coming to a position or into possession," especially in reference to a throne. Related: Accessional.