  The Einstein-Hilbert Lagrangian has no well-defined variational derivative
with respect to the metric. This issue has to be tackled by adding a suitable
surface term to the action, which is a peculiar feature of gravity. We also
know that null surfaces in spacetime exhibit (observer-dependent) thermodynamic
features. This suggests a possible thermodynamic interpretation of the boundary
term when the boundary is a null surface. For timelike/spacelike surfaces it is
easy to construct the boundary term but there are some subtleties in the case
of the null surface. The correct form of boundary term for null surfaces was
obtained recently from first principles. We show that this surface term, as
well as its variation, have direct thermodynamic interpretation in terms of a
heat density of null surfaces. The implications of the result are discussed.
