What is the movement caused within a fluid by the tendency of hotter, less dense material to rise and colder, denser material to sink called?

The movement described in the question is referred to as convection. This process occurs when warmer, less dense regions of a fluid (which can be a liquid or gas) rise while cooler, denser regions sink. This behavior creates a cyclical motion within the fluid, which is commonly observed in boiling water or atmospheric phenomena.

In simple terms, because warmer parts of the fluid tend to have a lower density, they will ascend, leading to a transfer of heat and material. As they rise, they may cool down, become denser, and then sink again, facilitating a continuous flow. This mechanism is vital in various natural processes, including weather patterns and ocean currents.

The other processes mentioned, such as conduction and radiation, refer to different mechanisms of heat transfer. Conduction involves direct contact transfer of heat through materials, while radiation involves the transfer of energy via electromagnetic waves. Diffusion describes the spreading of particles from areas of higher concentration to areas of lower concentration, which does not involve the movement driven by temperature differences as seen in convection.