1) when heat is transferred by the mass movement of molecules from one place to another.
2) The movement of heat by a moving liquid or gas. Convection results from the differences in the densities of a material at different temperatures. As a liquid or gas raises in temperature, it becomes less dense and thus lighter thereby rising above its cooler and denser counterparts, which in turn sink.
3) Process by which, in a fluid being heated, the warmer part of the mass will rise and the cooler portions sink. This is also a a component of the theory for continental drift, in that the circulating movements of crustal materials push the continents apart.source: [Journal of the Atmospheric Sciences. v 46, 1540, 1989.] [Journal of the Atmospheric Sciences. v 45, 2641, 1988.]