'applanation' originates from Latin via French, specifically from the Late Latin/Latin verb 'applanare', where the prefix 'ad-' (appearing as 'ap-' before 'p') meant 'to/toward' and 'planare' meant 'to make level/flat.'

'applanation' entered English from French 'applanation' and from Medieval/Latin 'applanare' (a- + 'planare'), evolving into the modern English noun 'applanation'.

Initially, it meant 'the act of making level or flat'; over time it retained that basic sense but also acquired a specialized medical sense referring specifically to flattening the cornea for measurement.