Answer:
When multiplication is defined over some domains, for every non-zero element X, in the domain, there exists a unique element Y, also in the domain such that X*Y = Y*X = 1 where 1 is the multiplicative identity. Such a value Y is written as X-1 or 1/X.
Note that a multiplicative inverse need not exist. For example, the set of integers is closed under multiplication, but most elements do not have an inverse within the set.