1 archaic : physician, surgeon 2 [from its former use by physicians for bleeding patients] : any of numerous carnivorous or bloodsucking usually freshwater annelid worms (class Hirudinea) that have typically a flattened lanceolate segmented body with a sucker at each end 3 : a hanger-on who seeks advantage or gain You are right Dave, it looks like a leech is a worm, not an insect.