In thermodynamics, the symbol commonly denoting heat transferred to a system is Q.
Heat is energy crossing a system boundary because of a temperature difference; it is not a substance stored inside an object. In the usual engineering sign convention, Q is positive when the system absorbs heat and negative when it releases heat. Heat is measured in joules, the SI unit of energy.
The symbol Q is associated with the quantity of heat, while the rate of heat transfer is commonly written as Q̇, pronounced “Q dot,” and measured in watts. The lowercase δ in δQ emphasizes that heat is a path-dependent transfer, not a property possessed by a system. By contrast, internal energy is a state function and is written with an exact differential, dU.
A common mix-up is confusing Q with W. W generally denotes work, while P denotes pressure and V denotes volume. In one common sign convention, the first law is ΔU = Q − W, linking heat and work to a system’s change in internal energy.