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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