tadpole

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

• noun The aquatic larva of a frog or toad, having gills, a long, laterally compressed tail, and in early stages, no limbs. During metamorphosis of a tadpole into an adult, legs and lungs develop, and the tail gradually disappears.

from The Century Dictionary.

• noun The larva of a batrachian, as a frog or toad, from the time it leaves the egg until it loses its gills and tail.
• noun The hooded merganser, Lophodytes cucullatus: doubtless so called from the apparent size of the head. See the quotation under moss-head.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) The young aquatic larva of any amphibian. In this stage it breathes by means of external or internal gills, is at first destitute of legs, and has a finlike tail. Called also polliwig, polliwog, porwiggle, or purwiggy.
• noun (Zoöl.), Local, U. S. The hooded merganser.
• noun (Zoöl.) See Forkbeard (a).

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

• noun A young toad or frog in its larval stage of development that lives in water, has a tail and no legs, and, like a fish, breathes through gills.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a larval frog or toad