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 mixture of different types 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.