contractile vacuole

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A membrane-bound organelle found in certain protists that pumps fluid in a cyclical manner from within the cell to the outside by alternately filling and then contracting to release its contents at various points on the surface of the cell. It functions in maintaining osmotic equilibrium.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun cell biology A vacuole that removes waste or excess water.