The term 'geometrical prefix' refers to a THREE-letter word that is used to indicate a geometric shape or form. The most common geometrical prefix is 'TRI', which is derived from the Greek word for 'THREE'. This prefix is used to refer to shapes or forms that have THREE sides, such as a triangle or a tricycle. It can also be used to refer to THREE-dimensional objects, such as a triangular prism. In addition, it is often used in mathematics to refer to THREE-dimensional figures, such as a triangle or a tetrahedron.