tadpole

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The limbless aquatic larva of a frog or toad, having gills and a long flat tail. As the tadpole approaches the adult stage, legs and lungs develop, and the tail gradually disappears. Also called polliwog.

Century Dictionary and Cyclopedia

1. n. The larva of a batrachian, as a frog or toad, from the time it leaves the egg until it loses its gills and tail. The name is chiefly the popular designation of the young of anurous batrachians, when the head and body form a rounded figure with a long tail, used like a fish's to swim with, and the creatures live in the water and breathe by gills. They gradually sprout their legs, drop or absorb their gills and tail, and come on land to breathe air. The term is also used of any other larvæ of amphibians in which the metamorphosis is less complete, as of newts, efts, or salamanders.
2. n. The hooded merganser, Lophodytes cucullatus: doubtless so called from the apparent size of the head. See the quotation under moss-head.

Wiktionary

1. n. 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.

GNU Webster's 1913

1. n. 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.
2. n. The hooded merganser.

WordNet 3.0

1. n. a larval frog or toad