  Boundary conditions defining a non-rotating isolated horizon are given in
Einstein-Maxwell theory. A spacetime representing a black hole which itself is
in equilibrium but whose exterior contains radiation admits such a horizon.
Inspired by Hamiltonian mechanics, a (quasi-)local definition of isolated
horizon mass is formulated. Although its definition does not refer to infinity,
this mass takes the standard value in a Reissner-Nordstrom solution.
Furthermore, under certain technical assumptions, the mass of an isolated
horizon is shown to equal the future limit of the Bondi energy.
