Frogbit \Frog"bit`\, n. (Bot.)
   (a) A European plant (Hydrocharis Morsus-ran[ae]), floating on still water and propagating itself by runners. It has roundish leaves and small white flowers.
   (b) An American plant (Limnobium Spongia), with similar habits.

Source: Webster's Revised Unabridged Dictionary (1913)