accession

A coming to; the act of acceding and becoming joined.

a king's accession to a confederacy

Increase by something added; that which is added; augmentation from without.

The only accession which the Roman empire received, during the first century of the Christian Aera, was the province of Britain.

1803, John Browne Cutting, “A Succinct History of Jamaica” in Robert Charles Dallas, The History of the Maroons, London: Longman and Rees, Volume 1, p. xli, […] armed vessels being provided, their crews were soon recruited by accessions from the needy or adventurous, the discontented or the bold.

Increase by something added; that which is added; augmentation from without.

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 spec

The act by which one power becomes party to engagements already in force between other powers.

accession to the European Union

To make a record of (additions to a collection); to add (something) to a collection (usually a museum's or archive's collection).