1. A respiratory or breathing organ or gill which is found in many mollusks: The cenidium is a structure which exists in bivalves and in many aquatic gastropods; such as, some freshwater snails and sea snails and also in some sea slugs.

Some aquatic gastropods possess one ctenidium while others have a pair of ctenidia.

A ctenidium is shaped like a comb or a feather, with a central part from which many filaments or plate-like structures protrude which are lined up in a row.

2. Etymology: The word is Latinized but is derived from the Greek ktenidion which means "little comb".