"Clay" is a "greasy part of the earth" that
remains that way even when wet and impermeable.
Clay is "soil composed chiefly of fine particles of
hydrous aluminum silicates and other minerals."

Merriam Webster adds the subsequent definition of clay as
"the human body as distinguished from the spirit."

There are many varieties and different colors of clay, each with its own properties. Clay is sometimes found in garden sub soils, about one yard deep, but generally it is dug out from quarries and sent to different industries, such as tile and brick factories, and pottery and ceramic plants.