Optimum Altitude– The best aircraft cruise altitude for a given weight and corresponding air temperature. An increase in temperature will lower the level of altitude capability and the optimum altitude.
In technical terms, optimum altitude is defined as the altitude at which the equivalent airspeed for a thrust setting will equal the square root of the coefficient of lift over the coefficient of drag.
From an operational perspective, the optimum altitude is the most economical altitude, which minimizes total costs related to the operation of the aircraft. Modern aircraft equipped with flight management computers (FMC’s) have the capability due to display the optimum altitude.