Knot

Knot (kt) is the standard unit of speed in aviation and maritime navigation, equal to one nautical mile per hour (1.852 km/h or 1.151 mph). Derived from the practice of counting knots on a rope trailed behind a ship.

Aviation speed terms:

IAS: Indicated Airspeed (instrument reading)
CAS: Calibrated Airspeed (corrected for instrument and position error)
TAS: True Airspeed (corrected for altitude and temperature)
GS: Ground Speed (speed over the ground, affected by wind)
Mach: Speed relative to the speed of sound