n 1 : the larval cyst of a tapeworm of the genus Echinococcus that usu. occurs as a fluid-filled sac containing daughter cysts in which scolices develop but that occas. forms a proliferating spongy mass which actively metastasizes in the host's tissues - called also hydatid cyst - see ECHINOCOCCUS1 2 a : an abnormal cyst or cystic structure : esp : HYDATIDIFORM MOLE b : HYDATID DISEASE .
Similar sounding terms: di·io·dide
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