Group (mathematics)
Inverse element: Every element in the group has another element in the group that is called its inverse. When the operation combines any element and its inverse, the answer is the identity element. Sometimes, the inverse is indicated by writing either a '-' in front, or by writing -1 in superscript, so the inverse of the element a is either -a or a-1 (it depends on how one writes the identity element; once again it is only a way of writing and does not say that elements must be numbers).