lemming

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of various small, thickset rodents, especially of the genus Lemmus, inhabiting northern regions and known for periodic mass migrations that sometimes end in drowning.

Century Dictionary and Cyclopedia

1. n. A rodent quadruped of the family Muridæ, subfamily Arvicolinæ, and one of the genera Myodes, Cuniculus, and Synaptomys (see these terms). The common European lemming, Mus lemmus of Linnæus, now Myodes lemmus, to which alone the name originally pertained, inhabits Norway, Sweden, Lapland, and other northern countries. It is about 5 inches long and of varied coloration. It is very prolific, and vast hordes periodically migrate down to the sea, destroying much vegetation in their path. So numerous are they at times, and so sudden is their appearance, that they were fabled to rain down from the clouds. Large numbers of rapacious quadrupeds and birds hang upon their line of march and materially diminish their numbers. These migrations are said to portend a hard winter. M. schisticolor is a plain slaty-gray species of Siberia. M. obensis is a bright rusty-brown species inhabiting arctic regions of both hemispheres and common in northwestern America. The lemming of the Hudson's Bay regions, Greenland, etc., is Cuniculus hudsonius or torquatus, a species of which turns snow-white in winter; it is also called hare-tailed mouse or rat, and by other names. A kind of false lemming, found in parts of the United States from Indiana and Kansas to Alaska, and also in British America, is Synaptomys cooperi. There are several other nominal species.
2. n. The North American lemmings, 8 or 10 species of which are now recognized, have been divided into two genera, Lemmus and Dicrostonyx, and the species once familiar as Myodes obensis is now Lemmus alascensis. The black-footed lemming, L. nigripes, is peculiar to St George Island, Bering Sea. The common northeastern species is D. hudsonius.

Wiktionary

1. n. A small Arctic and Subarctic rodent from any of six genera of similar rodents.
2. n. figuratively Any member of a group given to conformity or groupthink, especially a group poised to follow a leader off a cliff.

GNU Webster's 1913

1. n. (Zoöl.) Any one of several species of small arctic rodents of the genera Myodes and Cuniculus, resembling the meadow mice in form. They are found in both hemispheres.

WordNet 3.0

1. n. any of various short-tailed furry-footed rodents of circumpolar distribution