A convection cell is a phenomenon of fluid dynamics that occurs in situations where there are density differences within a body of liquid or gas. The convection usually requires a gravitational field but in microgravity experiments, thermal convection has been observed without gravitational effects being needed.

Fluids are materials that exhibit the property of flow. Both gases and liquids have fluid properties, and in sufficient quantity, even particulate solids such as salt, grain, and gravel show some fluid properties. When a volume of fluid is heated, it expands and becomes less dense and thus more buoyant than the surrounding fluid. The colder, denser fluid settles underneath the warmer, less dense fluid and forces it to rise. Such movement is called convection, and the moving body of liquid is referred to as a convection cell. This particular type of convection, where a horizontal layer of fluid is heated from below, is known as Rayleigh-Bénard convection.

Convection cells can form in any fluid, including the Earth's atmosphere (where they are called Hadley cells), boiling water, soup (where the cells can be identified by particles they transport, such as grains of rice), the ocean, the surface of the sun, or even a farmer's field, where large rocks have seemingly been forced to the surface over time in a process either analogous to or directly related to convection (the connection is not yet clear). The size of convection cells is largely determined by the fluid's properties, and they can even occur when the heating of a fluid is uniform.