Definitions of accession:

• noun:   the act of attaining or gaining access to a new office or right or position (especially the throne)

  Example: "Elizabeth's accession in 1558"

• noun:   agreeing with or consenting to (often unwillingly)

  Example: "Accession to such demands would set a dangerous precedent"

• noun:   something added to what you already have

  Example: "The librarian shelved the new accessions"

• noun:   (civil law) the right to all of that which your property produces whether by growth or improvement
• noun:   a process of increasing by addition (as to a collection or group)

  Example: "The art collectin grew through accession"

• noun:   the right to enter
• verb:   make a record of additions to a collection, such as a library