Convection is the heat transfer due to the bulk movement of molecules within fluids such as gases and liquids. Convection cannot take place in most solids because neither bulk current flows nor significant diffusion of matter can take place. Diffusion of heat takes place in rigid solids, but that is called heat conduction. Convection, additionally may take place in soft solids or mixtures where solid particles can move past each other.
Sourced from Wikipedia.