Critical altitude

The maximum altitude at which, in standard atmosphere, it is possible to maintain, at a specified rotational speed, a specified power or a specified manifold pressure. Unless otherwise stated, the critical altitude is the maximum altitude at which it is possible to maintain, at the maximum continuous rotational speed, one of the following:
  1. The maximum continuous power, in the case of engines for which this power rating is the same at sea level and at the rated altitude.
  2. The maximum continuous rated manifold pressure, in the case of engines, the maximum continuous power of which is governed by a constant manifold pressure.
(FAR definition).

The maximum altitude under standard atmospheric conditions at which a turbocharged engine can produce its rated horsepower. (PHAK definition).

Source: 14 CFR Federal Aviation Regulations, Pilot Handbook of Aeronatical Knowledge