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.

A sponge is a sea animal with a soft round body made of natural sponge.

A sponge is a piece of sponge that you use for washing yourself or for cleaning things.

If you sponge something, you clean it by wiping it with a wet sponge.

A sponge is a light cake or pudding made from flour, eggs, sugar, and sometimes fat.

If you say that someone sponges off other people or sponges on them, you mean that they regularly get money from other people when they should be trying to support themselves.