Convection is the movement of molecules within fluids (i.e. liquids, gases and rheids). It cannot take place in solids, since neither bulk current flows or significant diffusion can take place in solids.Convection is one of the major modes of heat transfer and mass transfer. Convective heat and mass transfer take place through both diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid. In the context of heat and mass transfer, the term "convection" is used to refer to the sum of advective and diffusive transfer.

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Reference by http://www.cheresources.com/convection.shtml via wikipedia