Geometric or tapeline altitude is the physical, linear altitude measured from mean sea level.

Geopotential altitude is the geometric altitude adjusted for variation of gravity. For the standard atmosphere model, Hp (pressure altitude) and H (Geopotential altitude) are identical by definition if the sea level pressure is exactly the standard atmosphere value and that temperature is precisely standard day at all altitudes (not just at the altitude being considered).

In the early days of flight, the radar technology was not available to measure altitude onboard an aircraft. However, the aircrew could measure the outside ambient pressure. A standard atmosphere was defined which allowed the computation of an altitude that was proportional to the measured ambient pressure. That altitude is the pressure altitude.

International Standard Atmosphere Deviation

Temperature Ratio T/T_o, the ratio of the air temperature to the temperature at sea level in the standard atmosphere.

Density Ratio ρ/ρ_o, the ratio of the air density to the densit at sea level in the standard atmosphere.

Pressure Ratio P/P_o, the ratio of the air pressure to the pressure at the sea level in the standard atmosphere.

Calibrated Airspeed in Knots (Nautical Miles per Hour)

Calibrated Airspeed is the indicated airspeed with position error correction. The indicated airspeed is the measured speed at the actual pitot-static probe.

Equivalent Airspeed in Knots (Nautical Miles per Hour)

KEAS takes into account the effect of compressibility. It is often used for structural analysis.

True Airspeed in Knots (Nautical Miles per Hour)

True airpssed is the physical speed of the vehicle relative to the moving air mass. It is used to calculate the Mach number.

Outside Air Temperature or Static Air Temerature.