What characteristic defines convection as a method of heat transfer?

Convection is characterized primarily by the movement of fluids, which can include gases and liquids. This process involves the transfer of heat through the fluid's motion, where warmer regions of the fluid rise due to decreased density and cooler regions sink. This creates a continuous circulation pattern, allowing for efficient heat distribution throughout the fluid.

In contrast, direct contact only refers to conduction, where heat is transferred between materials that are in direct physical contact. Electromagnetic waves define radiation, which is another method of heat transfer that does not require a medium. Lastly, solids are typically involved in conduction, as heat is conducted through the material rather than through fluid movement. Therefore, the defining characteristic of convection as a method of heat transfer is indeed the movement of fluids.