Speed measure is a term used to describe the rate at which something is moving. The three crossword solutions for this term are MACH (4 letters), KNOT (4 letters) and FSTOP (5 letters). MACH stands for Mach number, which is a unit of speed equal to the speed of sound in air. KNOT is a unit of speed equal to one nautical mile per hour. FSTOP is a unit of measurement used in photography to indicate the size of the aperture of a camera lens.