Noun: 1. a primitive sedentary aquatic invertebrate with a soft porous body that is typically supported by a framework of fibers or calcareous or glassy spicules. Sponges draw in a current of water to extract nutrients and oxygen. 2. a piece of a soft, light, porous substance originally consisting of the fibrous skeleton of an invertebrate but now usually made of synthetic material. Sponges absorb liquid and are used for washing and cleaning.