Medical Definition of leech
: any of numerous carnivorous or bloodsucking annelid worms that comprise the class Hirudinea, that typically have a flattened segmented lance-shaped body with well-marked external annulations, a sucker at each end, a mouth within the anterior sucker, and a large stomach with pouches of large capacity at the sides, that are hermaphroditic usually with direct development, and that occur chiefly in freshwater although a few are marine and some tropical forms are terrestrial—see medicinal leech
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