True airspeed (TAS)
The airspeed of an aircraft relative to undisturbed air. True airspeed is equal to equivalent airspeed multiplied by (ρ0/ρ)1/2. (FAR definition).
Actual airspeed, determined by applying a correction for pressure altitude and temperature to the CAS. (PHAK definition)
Source: 14 CFR Federal Aviation Regulations §1, Pilot Handbook of Aeronatical Knowledge