Convection
Definition
Heat carried by moving fluid — hot air or water rises, cool fluid sinks, and the circulation transports energy.
Technical definition
Heat transfer by bulk motion of a fluid, driven by buoyancy: heated fluid expands, becomes less dense, and rises, setting up convection currents. Heat loss from surfaces follows Newton's law of cooling.
Example
A radiator warms a room without any fan: air touching it heats up, rises to the ceiling, cools, sinks across the room, and returns — a slow loop that eventually warms every corner.
Convection cannot happen in solids. Only fluids (liquids and gases) convect, because convection requires the material itself to flow. The handle of a hot pan warms by conduction through solid metal; the air above the pan warms by convection. Saying 'heat convects through the wall' is a category error — walls conduct.