The pressure generated by the aqueous humor in the eyes.
The normal intraocular pressure is statistically between 10-21 mmHg and fluctuates by 5 mmHg during the course of the day. In glaucoma patients, this value is usually above the 21 mmHg limit.
https://de.wikipedia.org/wiki/Augeninnendruck (Wikipedia CC-by-sa-3.0)