Aquaporin allows water molecules to move very rapidly across a plasma membrane. what would be the best definition of this process?

This is an example of facilitated diffusion using a channel. Diffusion is the process of an object moving across a concentration gradient, usually from areas of higher to lower concentration in membranes. In this case, the diffusion is "facilitated" by the aquaporin molecules and water is allowed to enter or exit the membrane based on concentration level.