accession (n.) Look up accession at Dictionary.com
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.