Rigid Body is basically a rigid object, however, unlike fuel, elastic, string or weak, it does not react with anything. It reacts very much like the one in OE-Cake, Except for the color change.
In Real LifeEdit
Rigid is an adjective, however rigid object is a broad noun which can be an item which falls, and not able to change shape easily (kind of like paper). Examples: wood, glass, hard plastic, metal, etc