Carnot Cycle
Definition
The ideal, most efficient possible heat engine cycle — the theoretical limit no real engine can beat.
Unit: dimensionless
Technical definition
A reversible thermodynamic cycle of two isothermal and two adiabatic processes. Its efficiency η = 1 − T_C/T_H (temperatures in kelvin) sets the maximum possible efficiency for any heat engine operating between those temperatures — a consequence of the second law.
Example
An engine between 600 K and 300 K cannot exceed 50% efficiency — even ideally. Real car engines (~25%) fall well short due to friction and irreversibility.
The Carnot limit uses absolute (kelvin) temperatures — plugging in Celsius gives nonsense. And it is a limit for heat engines specifically, not for all energy conversion (fuel cells and electric motors play by different rules).