heat
Heat is energy on the move from a hotter thing to a colder thing, simply because of the gap in their temperatures. Hold a mug of cocoa and your cold fingers warm up: energy is flowing from the hot drink into your skin, with no pushing or pulling involved — just contact and a temperature difference. That spontaneous, downhill flow of energy is what physicists mean by heat.
Notice that heat is a process, not a possession. It is energy in transit across a boundary, and it only exists while the crossing is happening. Once the cocoa and your hand reach the same temperature, no heat flows anymore, even though both still hold plenty of internal energy. In equations it is written q, counted as positive when energy flows into the system and negative when it leaves.
Heat is a path function: how much flows depends on the route the change takes, not just on the start and end states. Because of this, it is wrong to say an object "has" a certain amount of heat. It has internal energy; heat is one of the two ways — work being the other — that this energy can cross the system's edge.
Drop a hot iron horseshoe into a bucket of cool water and the water warms while the iron cools. Energy flows as heat from iron to water until both settle at one temperature.
Heat always flows from hot to cold on its own, never the reverse without help.
In everyday speech "heat" often means the same as temperature or warmth, but in thermodynamics it has the strict meaning of energy in transit driven by a temperature difference. Temperature tells you how hot something is; heat tells you how much energy is crossing because of that hotness gap.