The Collaborative International Dictionary of English v.0.48:

Butterwort \But"ter*wort`\, n. (Bot.)
   A genus of low herbs (Pinguicula) having simple leaves
   which secrete from their glandular upper surface a viscid
   fluid, to which insects adhere, after which the margin
   infolds and the insects are digested by the plant. The
   species are found mostly in the North Temperate zone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

butterwort
    n 1: any of numerous carnivorous bog plants of the genus
         Pinguicula having showy purple or yellow or white flowers
         and a rosette of basal leaves coated with a sticky
         secretion to trap small insects