What does accession mean?
The definition of accession
Ac*ces”sion, n.
Etym: [L. accessio, fr. accedere: cf. F. accession. See Accede.]

1 A coming to; the act of acceding and becoming joined; as, a king’s accession to a confederacy.
2 Increase by something added; that which is added; augmentation from without; as, an accession of wealth or territory. The only accession which the Roman empire received was the province of Britain. Gibbon.
3 (Law)

(a) A mode of acquiring property, by which the owner of a corporeal substance which receives an addition by growth, or by labor, has a right to the part or thing added, or the improvement (provided the thing is not changed into a different species). Thus, the owner of a cow becomes the owner of her calf.
(b) The act by which one power becomes party to engagements already in force between other powers. Kent.

4 The act of coming to or reaching a throne, an office, or dignity; as, the accession of the house of Stuart; — applied especially to the epoch of a new dynasty.
5 (Med.)
The invasion, approach, or commencement of a disease; a fit or paroxysm.

Synonyms
— Increase; addition; augmentation; enlargement.