Convection
Convection is heat transfer by the movement of a fluid, as warmer, less dense liquid or gas rises and cooler, denser fluid sinks.
Definition
Convection is heat carried along by moving liquid or gas: warm fluid expands, becomes less dense, and rises, while cooler, denser fluid sinks to take its place, forming a convection current that keeps carrying heat around. Convection cannot happen in solids; natural convection is driven by these density differences, while forced convection uses a fan or pump to move the fluid. Convective transfer from a surface follows Newton's law of cooling, $\dot{Q} = hA(T_s - T_f)$, where $h$ is the convective heat transfer coefficient that depends on fluid properties, flow speed, and geometry, with natural convection beginning once buoyancy overcomes viscosity and thermal diffusion, a balance captured by the Rayleigh number.
Example
Air above a radiator warms, rises to the ceiling, and cooler air near the floor slides over to replace it, warming the whole room; in a pot of water on a stove, the bottom layer heats to near $100^\circ\text{C}$ and rises while cooler $60^\circ\text{C}$ water sinks, producing the rolling motion visible in boiling water. A $0.5\ \text{m}^2$ radiator surface at $60^\circ\text{C}$ in $20^\circ\text{C}$ room air has $h \approx 10\ \text{W/(m}^2\cdot\text{K)}$ for natural convection, giving $\dot{Q} = 10 \times 0.5 \times 40 = 200\ \text{W}$, but adding a fan raises $h$ to about $50$ and lifts the same radiator's output to $1{,}000\ \text{W}$.
Key Insight
Convection is why the top bunk is always hotter than the bottom bunk and why smoke rises from a campfire, and on a planetary scale it drives sea breezes, weather, ocean currents, Earth's mantle motion, and energy transport in the outer layers of the Sun. It is really conduction with a conveyor belt: heat still enters the fluid by conduction at the surface, but the moving fluid carries it away far faster than conduction alone could.