The act or process of flattening something; in medicine, the measurement of eye pressure using a flattening device (applanation tonometry).
From Latin applanatio, derived from applanare (to flatten). The -ation suffix converts the verb into a noun describing the action. In ophthalmology, it took on a specific technical meaning in the 20th century.
Eye doctors use applanation tonometry to test for glaucoma—they literally flatten your cornea with a gentle instrument to measure the pressure inside your eye. It sounds uncomfortable but it's quick, and the concept is brilliant: the more pressure inside, the more force needed to flatten the surface!
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