The Rayleigh-Taylor instability (RT instability) occurs at the interface between two fluids of different density when the less dense fluid is pushing against the more dense fluid, e.g., within a gravitational field, a more dense fluid "on top" of a less dense fluid. If any bit of the lighter fluid rises, pushing the boundary at one location, it will continue. The instability is the trigger for a common type of convection.
It is involved in astrophysical phenomena such as stellar structure, supernovae, and the interstellar medium and star formation.
It is named for Lord Rayleigh and Geoffrey Ingram Taylor.