Aqueous humor is the watery, alkaline fluid that fills the space between the cornea and the vitreous humor of the vertebrate eye. In it lie the iris and the lens. The aqueous humor is continuously secreted, by the ciliary body, and absorbed at a constant rate of about 5 milliliter (1 teaspoon) per day. It helps maintain a steady pressure in the anterior (front) chamber of the eye and also nourishes those parts of the front of the eye that are not served by blood vessels, such as the lens, the back surface of the cornea, and the trabecular meshwork.

Aqueous humor flows into the front of the eye through the pupil and drains out through the trabecular meshwork, a spongy mass of tiny canals located in the drainage angle. The drainage angle is located between the iris and the cornea, where the iris meets the sclera. Aqueous humor is returned into the bloodstream through a tiny channel, called Schlemm's canal, located under the trabecular meshwork.