  To every axi-symmetric isolated horizon we associate two sets of numbers,
$M_n$ and $J_n$ with $n = 0, 1, 2, ...$, representing its mass and angular
momentum multipoles. They provide a diffeomorphism invariant characterization
of the horizon geometry. Physically, they can be thought of as the `source
multipoles' of black holes in equilibrium. These structures have a variety of
potential applications ranging from equations of motion of black holes and
numerical relativity to quantum gravity.
