 Gibbons–Hawking–York boundary term

The Gibbons–Hawking–York boundary term makes the Einstein--Hilbert variational problem well posed when the induced boundary metric is fixed. It is the integral of the trace of the boundary's extrinsic curvature with the conventional normalization and sign.

The Gibbons–Hawking–York (GHY) boundary term is an important concept in the context of general relativity and gravitational action principles, particularly when dealing with the Einstein-Hilbert action, which describes the dynamics of gravity.