a ‧ pos ‧ ta ‧ sy /əˈpɒstəsi $ əˈpɑː-/ BrE AmE noun [uncountable]
[ Date: 1300-1400 ; Language: Late Latin ; Origin: apostasia , from Greek , from aphistasthai 'to revolt' , from apo- ( ⇨ ↑ apocalypse ) + histasthai 'to stand' ]
formal when someone suddenly stops believing in a religion or supporting a political party