The Carnot cycle is the ideal, fully reversible cycle that sets the theoretical maximum efficiency any heat engine can achieve while operating between a hot reservoir at temperature T₁ and a cold reservoir at temperature T₂. Its efficiency depends only on those two temperatures (η = 1 − T₂/T₁), which is why it serves as the benchmark against which real engines are compared.

The cycle is built from four reversible processes, arranged as two isothermal and two adiabatic steps alternating with one another:

1. Isothermal expansion — the gas absorbs heat from the hot reservoir at constant temperature T₁ while doing work.
2. Adiabatic (isentropic) expansion — no heat is exchanged; the gas continues to expand and its temperature falls from T₁ to T₂.
3. Isothermal compression — the gas rejects heat to the cold reservoir at constant temperature T₂.
4. Adiabatic (isentropic) compression — no heat is exchanged; the gas is compressed and its temperature rises back from T₂ to T₁, returning it to the start.

So the two types of process that make up the cycle are the isothermal and the adiabatic.

Why the other pairings are wrong: Any combination naming isochoric (constant volume) or isobaric (constant pressure) steps describes a different cycle — for example, isothermal-plus-isochoric appears in the Stirling cycle and adiabatic-plus-isochoric in the Otto cycle. None of these constant-volume or constant-pressure steps occur in the Carnot cycle, so only isothermal and adiabatic is correct.