ac ‧ ces ‧ sion /əkˈseʃ ə n/ BrE AmE noun
1 . [uncountable] an official process in which someone becomes king, queen, president etc ⇨ succession
accession of
the accession of James I
accession to power/to the throne (=the act of becoming king, queen, president etc)
2 . [uncountable] formal the act of agreeing to a demand
3 . [countable] technical an object or work of art that is added to a collection of objects or paintings