  Extending the notion of indispensable binomials of a toric ideal, we define
indispensable monomials of a toric ideal and establish some of their
properties. They are useful for searching indispensable binomials of a toric
ideal and for proving the existence or non-existence of a unique minimal system
of binomials generators of a toric ideal. Some examples of indispensable
monomials from statistical models for contingency tables are given.
