1. A water channel.
2. Any of a family of proteins found in the plasma membranes of plant and animal cells and forming a functional component of water channels.

Aquaporins form pores in the membranes of cells and selectively conduct water molecules through the membrane, while preventing the passage of ions (such as sodium and potassium) and other small molecules.

Aquaporins are typically composed of identical subunit proteins. Water molecules traverse the narrowest portion of the water channel in single file. The presence of water channels increases the permeability of membranes to water by as much as ten-fold.

Aquaporins keep people from dying of dehydration by reabsorbing 99% of the water in the kidneys.