What has to be the shortest definition in the dictionary is that of Waeg. The following two words are typical, sometimes its just one word:

Waeg – 1 dictionary result

Waeg\, n. (Zo["o]l.) The kittiwake. [Scot.]
Webster’s Revised Unabridged Dictionary, © 1996, 1998 MICRA, Inc.

Kittiwake has four similar definitions from Dictionary.com Unabridged

kit-ti-wake
–noun either of two small, pearl-gray gulls of the genus Rissa, the black-legged R. tridactyla of the North Atlantic and the red-legged and red-billed R. brevirostris, of the Bering Sea, both nesting on narrow cliff ledges and having a rudimentary hind toe.

Origin:
1655–65; imit.

kit·ti·wake Pronunciation Key
n. Either of two gray gulls (Rissa tridactyla or R. brevirostris) of northern Atlantic regions, that winter on the open ocean and nest in colonies on sea cliffs.
[Perhaps imitative of its cry.]

Kittiwake
Kit”ti*wake\, n. (Zo["o]l.) A northern gull (Rissa tridactyla), inhabiting the coasts of Europe and America. It is white, with black tips to the wings, and has but three toes.

kittiwake noun
small pearl-grey gull of northern regions; nests on cliffs and has a rudimentary hind toe