Answer: Applanation is a method for checking eye pressure. The idea is that you measure the amount of force it takes to flatten a predetermined area of cornea, and use this to estimate the internal eye pressure.
We use the Goldmann applanation tonometer to do this. It’s a device mounted on the slit-lamp that has an applanation tip that presses against the surface of the cornea. A prism in the tip allows you to adjust the force applied to flatten the exact amount of cornea. Fleuroscein is also placed in the eye in order to see the applanation mires better.