  Boundary conditions defining a generic isolated horizon are introduced. They
generalize the notion available in the existing literature by allowing the
horizon to have distortion and angular momentum. Space-times containing a black
hole, itself in equilibrium but possibly surrounded by radiation, satisfy these
conditions. In spite of this generality, the conditions have rich consequences.
They lead to a framework, somewhat analogous to null infinity, for extracting
physical information, but now in the \textit{strong} field regions. The
framework also generalizes the zeroth and first laws of black hole mechanics to
more realistic situations and sheds new light on the `origin' of the first law.
Finally, it provides a point of departure for black hole entropy calculations
in non-perturbative quantum gravity.
