The term PRISM can refer to two different things. The first is a SOLID object with five sides, also known as a pentagonal prism. It is a three-dimensional shape with two parallel faces that are the same size and shape, and the other three faces are rectangular. The second definition of PRISM is a type of geometry, which is a branch of mathematics that studies the properties of shapes and figures. GEO is an abbreviation for this type of geometry.