Osmosis

Osmosis is a process which the movement of water molecules from a solution of higher water potential to a solution of lower water potential through a partially permeable membrane, and is present in animal or plant cells.
http://www.vivo.colostate.edu/hbooks/cmb/cells/pmemb/osmosis.html

(An example of osmosis would be dipping a sponge into water, because there is a higher concentration outside, so the water goes in and makes the sponge expand.)