A 4-sided polyhedral die, shaped like a tetrahedron. It is the only commonly encountered die which does not present a face directly upward when at rest on a flat surface. Thus the usual way of numbering and reading a die does not work, and they require some other, slightly more complex method.
May be humorously referred to as a "caltrop", due to the shared tetrahedral shape and ability to inflict pain if inadvertently stepped on.
