In heat-engine diagrams, what is the usual name for the hotter reservoir?

In heat-engine diagrams, the hotter reservoir is usually called the hot reservoir.

A heat engine operates between two thermal reservoirs: it absorbs heat from the hot reservoir, converts part of that energy into work, and releases the remainder to the cold reservoir or heat sink. The reservoirs are idealized bodies large enough to maintain nearly constant temperatures while exchanging heat.

“Heat source” and “hot reservoir” are closely related terms, but the latter is the standard label in thermodynamic diagrams. The working fluid is not a reservoir: it is the substance that undergoes the cycle and carries energy through processes such as expansion and compression. Likewise, a heat sink is normally the colder destination, not the hotter source.

The distinction matters in the Carnot efficiency formula, η = 1 − T_C/T_H, where T_H is the absolute temperature of the hot reservoir and T_C is that of the cold reservoir. Real engines always fall below this ideal limit because of irreversibility.