What is convection in terms of heat transfer?

Convection refers specifically to the transfer of heat through the movement of fluids, which can be liquids or gases. In convection, heat is distributed through the fluid as it circulates, allowing warmer areas to rise and cooler areas to sink, thus creating a flow that enhances heat transfer. This process is commonly observed in scenarios such as boiling water, where hot water rises to the surface and cooler water descends, or in atmospheric processes like wind.

The other options describe different mechanisms of heat transfer: direct contact is characteristic of conduction, where heat is transferred between materials that are in direct physical contact. Radiation refers to heat transfer through electromagnetic waves, which does not require a medium to travel through, while phase changes involve processes like melting or boiling, where heat transfer is associated with changing states of matter rather than movement. Thus, convection is accurately defined as the heat transfer method that involves a moving fluid.