A molecular hydrogen dissociation front (H2 dissociation front or in context, just dissociation front) is a boundary in a cloud or atmosphere between molecular and atomic hydrogen. If lower depths (inner portions) of a cloud or atmosphere are hotter, then the dissociation front constitutes the boundary around the volume sufficiently hot to dissociate hydrogen molecules. The boundary can have a significant impact on opacity, thus marks a difference in cooling ability and potentially whether the gas is convective or radiative, on its cooling time, and more. In the case of planet formation, it is a factor in the ultimate amount of atmosphere retained by the planet.
An ionization front is a similar boundary between gas hot enough to remain ionized versus remaining neutral.