Plasticity (in ceramics) is a property exhibited by soft clay. Force exerted effects a change in shape and the clay exhibits no tendency to return to the old shape. Elasticity is the opposite.
This term is used in reference to clays (or more often bodies which are blends of clay, feldspar and silica particles) and their ability to assume a new shape without any tendency to return to the old (elasticity). Plasticity is a product of the electrolytic character of flat clay particles (they have opposite charges on the faces and edges), it gives them an affinity for water, water becomes both the glue holding particles together and the lubricant that imparts the plasticity. There are many finer points to understanding the dynamics of plasticity and it is difficult to measure using test equipment. It is only by a lot of hard work testing many material combinations that one can start to get an understanding of the complex factors that interplay to create the different kinds of plasticity we can detect and how these relate to the other properties of the body or material (e.g. dry strength, drying shrinkage, hardness, LOI, etc.). When one understands his/her materials well (especially the ball clays, kaolins and bentonites available to him), bodies of more plasticity that have less drying shrinkage and better drying performance can be created.