relativistic speed

Relativistic speed is a speed at which the laws of relativity become significant, i.e., a speed that is more than an insignificant fraction of the speed of light (c). Non-relativistic speed refers to speeds too slow to qualify. Ultrarelativistic speed means very close to c. For example, 0.25c could be considered relativistic and 0.999c, ultrarelativistic. (Such speeds are often expressed as fractions of c.) Some of the theorizing of astrophysical phenomena consists of figuring out means by which particles observed to be moving at relativistic or ultrarelativistic speed might have achieved that velocity. Speeds close to c are also cited by giving their Lorentz factor, a number which is roughly 1 for non-relativistic speeds, and rises without bound the closer a speed is to c.

(The term relativistic velocity means roughly the same thing as relativistic speed, but a velocity is a vector that additionally indicates the direction of motion, and the relativistic speed is more precisely the vector magnitude of the relativistic velocity.)