  A set of boundary conditions defining an undistorted, non-rotating isolated
horizon are specified in general relativity. A space-time representing a black
hole which is itself in equilibrium but whose exterior contains radiation
admits such a horizon. However, the definition is applicable in a more general
context, such as cosmological horizons. Physically motivated, (quasi-)local
definitions of the mass and surface gravity of an isolated horizon are
introduced and their properties analyzed. Although their definitions do not
refer to infinity, these quantities assume their standard values in the static
black hole solutions. Finally, using these definitions, the zeroth and first
laws of black hole mechanics are established for isolated horizons.
