accession: a process of increasing by addition (as to a collection or group); "the art collectin grew through accession" accession: (civil law) the right to all of that which your property produces whether by growth or improvement accession, addition: something added to what you already have; "the librarian shelved the new accessions"; "he was a new addition to the staff" accession, assenting: agreeing with or consenting to (often unwillingly); "accession to such demands would set a dangerous precedent"; "assenting to the Congressional determination" entree, access, accession, admittance: the right to enter accession, rise_to_power: the act of attaining or gaining access to a new office or right or position (especially the throne); "Elizabeth's accession in 1558" accession: make a record of additions to a collection, such as a library