A flow driven by horizontal pressure gradients, due typically to the flow of a fluid of one density intruding into an ambient fluid of different density. In non-rotating fluid, the head of the current travels with uniform horizontal speed C (g'H)^(1/2) in which g' is the reduced gravity, H is the current depth, and C is a constant of order unity. Sea breezes, landslides, and some instances of cold water formation are common manifestations of gravity currents in geophysical circumstances.