Lemming definition, any of various small, mouselike rodents of several genera including Lemmus, Myopus, and Dicrostonyx, of far northern regions, as L. lemmus, of Norway, Sweden, etc., noted for periodic mass migrations that sometimes involve crossing bodies of water. Incidental drownings that have occurred during such passage gave rise to the myth of mass suicides among supposedly frenzied lemmings jumping from cliffs into the sea. See more.