A term used by potters and in the ceramic industry. It refers to the earthenware, stoneware or porcelain that forms the piece (as opposed to the engobe and covering glaze).
For potters and in industry, the term "clay" is not specific enough, thus the term "body" is used. Clays are ingredients that can be found in glazes, engobes and in the body (but obviously bodies have a higher clay proportion than glazes). Thus there is danger of ambiguity when one refers to the body as "the clay". Notwithstanding this, the term "clay" is often used when referring to the body used as a commodity that needs to be purchased (e.g. "I am going to buy some clay"). Likewise, clays that are ingredients in bodies and glazes are referred to by their mineral names in commerce (e.g. kaolin, ball clay, bentonite).
A typical stoneware or porcelain body is mixture of raw and/or refined clays with likely additions of silica as a filler (and thermal expansion increaser) and feldspar as a flux. In vitrified bodies the larger grains of quartz and other refractory particles remain unmelted while the clay particles go into solution in the feldspar glass. In these, a transformation occurs in the matrix, a new amorphous solid is born having grains, crystals and a glassy glue binding the whole mass together. Sintered and earthenware bodies have strength by virtue of whatever particle bonding mechanism occur. When potters or technicians refer to the fired clay or porcelain matrix under the glaze they also call it "the body" (e.g. the body is vitrified, the glaze crazes on this body).
What is clay? How is it different that regular dirt? For ceramics, the answer lies on the microscopic level with the particle shape, size and how the surfaces interact with water.