trep·a·na·tion \ˌtrepəˈnāshən\noun (-s) Etymology: Middle English trepanacioun, from Middle French trepanation, from trepan (from Medieval Latin trepanum) + -ation 1.: the act or process of perforating a skull with a surgical instrument 2.: a hole in the skull produced surgically < hundreds of crania showing trepanations … have received close scrutiny by prehistorians — W.T.Corlett >