platypus

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

• noun A semiaquatic egg-laying mammal (Ornithorhynchus anatinus) of eastern Australia and Tasmania, having a broad flat tail, webbed feet, a snout resembling a duck's bill, and in the male, venomous spurs on the hind legs.

from The Century Dictionary.

• noun In entomology, a genus of xylophagous beetles of the family Scolytidæ: synonymous in part with Bostrychus.
• noun In mammalogy: A genus of monotremes, now called Ornithorhynchus.
• noun [lowercase] The species of this genus; the duckbilled platypus. See cut under duckbill.
• noun In ornithology, a genus of sea-ducks of the family Anatidæ and the subfamily Fuligulinæ: synonymous with Fulix.

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

• noun (Zoöl.) The duck mole. See under duck.

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

• noun An egg-laying, semi-aquatic mammal with a bill resembling that of a duck, that has a mole-like body, a tail resembling that of a beaver, a waterproof pelt, and flat webbed feet — males have poisonous spurs on the inside of the back legs; Ornithorhynchus anatinus

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

• noun small densely furred aquatic monotreme of Australia and Tasmania having a broad bill and tail and webbed feet; only species in the family Ornithorhynchidae