True airspeed (TAS) is defined as:

True airspeed (TAS) is defined as the speed of an aircraft through the air mass. This measurement is crucial for pilots as it gives them an accurate representation of how quickly the aircraft is moving through the surrounding air. It takes into account the effects of altitude and temperature, allowing for precise calculations necessary for navigation, flight planning, and performance assessment.

Unlike indicated airspeed, which is what the aircraft's speedometer displays and can be affected by errors such as instrument calibration, TAS provides valuable information about the aircraft’s actual speed in relation to the air it is traveling through. This distinction is essential for understanding flight dynamics and ensuring safe flight operations.

The other options illustrate different types of speed measurements used in aviation: indicated airspeed pertains to the reading on the aircraft's dashboard, ground speed is the speed relative to the Earth's surface (which can be affected by wind), and calibrated airspeed refers to indicated airspeed corrected for instrument and position errors, not adjusted for altitude or temperature. Each serves specific purposes, but TAS remains the fundamental measure of speed through the air mass.