Real Dictionary

sponge
countable noun: Sponge is a very light soft substance with lots of little holes in it, which can be either artificial or natural. It is used to clean things or as a soft layer.

sponge in American English
any of a phylum (Porifera) of simple, aquatic, sessile animals having a porous structure and a tough, often siliceous or calcareous, skeleton
to gather sponges from the sea
to use a sponge on so as to dampen, wipe clean, etc.
intransitive verb: to take in or soak up liquid by absorption
noun: any aquatic, chiefly marine animal of the phylum Porifera, having a porous structure and usually a horny, siliceous or calcareous internal skeleton or framework, occurring in large, sessile colonies
transitive verb: to wipe or rub with or as with a wet sponge, as to moisten or clean

sponge in British English
noun: any multicellular typically marine animal of the phylum Porifera, usually occurring in complex sessile colonies in which the porous body is supported by a fibrous, calcareous, or siliceous skeletal framework