Hidrocystomas are cysts of sweat ducts, usually on the eyelids.:664 They are not tumours (a similar sounding lesion called hidroadenoma is a benign tumour).
There are three types of "sweat" glands: True sweat glands or eccrine glands, sebaceous glands which have an oily secretion around hair follicles and apocrine glands which have more oily product than eccrine glands and are found on the face, armpit and groin.
Hidrocystomas usually arise from apocrine glands. They are also called Cysts of Moll or sudoriferous cysts. There may be a type of hidroadenoma that arises from eccrine glands but these are uncommon.
Other related conditions on the eyelids include chalazion ( a granulomatous reaction to sebaceous glands on the eyelid), lacrimal duct cysts (cysts related to tear ducts) and nasolacrimal duct cysts (the nasolacrimal duct drains tears into the nose via a punctum on the lower eyelid).