cys·toid I. \ˈsiˌstȯid\adjective Etymology: International Scientific Vocabulary cyst- + -oid; probably originally formed as French cystoïde 1.: like a bladder 2.[New Latin Cystoidea]: cystoidean II. noun (-s) 1.: a cystoid structure; specifically: a mass resembling a cyst but lacking a membrane 2.: cystoidean