/ ækˈseʃn; NAmE / noun accession (to sth)
1.
[ U ] the act of becoming a ruler of a country :
the accession of Queen Victoria to the throne
—see also accede
2.
[ U ] the act of becoming part of an international organization :
the accession of new member states to the EU in 2004
the new accession states of the EU
3.
[ C ] ( technical ) a thing that is added to a collection of objects, paintings, etc. in a library or museum
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WORD ORIGIN
late 16th cent. (in the general sense something added ): from Latin accessio(n-) , from the verb accedere approach, come to from ad- to + cedere give way, yield.