Stories We Like: A Guide to the Comma
1620s, from Medieval Latin annexiationem (nominative annexatio) "action of annexing," noun of action from past participle stem of annexare (see annex). The Middle English noun form was annexion "union; joining; territory acquired" (mid-15c.).
a formal act whereby a state proclaims its sovereignty over territory hitherto outside its domain. Unlike cession, whereby territory is given or sold through treaty, annexation is a unilateral act made effective by actual possession and legitimized by general recognition