Clay body (noun) refers to the actual clay mixture that is used in forming objects. It might only have one specific type of clay in it, but it is more likely to consist of a mix of various kinds of clay.
Other additives may also be introduced into the mixture. Each specific ingredient used is included to give the clay particular attributes. For example, ingredients may be added to aid plasticity, to lower or raise the clay body's firing temperature, or change its fired coloration. Clay bodies come in a wide spectrum of colors, from white to red, to black, and many colors in between.