Density altitude

Pressure altitude corrected for nonstandard temperature.

As taught in Density Altitude and Aircraft Performance (Weight, Balance and Aircraft Performance) — a free lesson with cited FAA sources.

Pressure altitude corrected for temperature; the altitude the airplane "feels".

As taught in The Atmosphere: Stability, Moisture and Density Altitude (Weather Theory) — a free lesson with cited FAA sources.

Pressure altitude corrected for temperature; high values lengthen takeoff distance.

As taught in Runway, Performance and Fuel Requirements (Preflight Planning and Required Information) — a free lesson with cited FAA sources.