relativistic speed

Relativistic speed is a speed at which the laws of relativity become significant, i.e., more than an insignificant fraction of the speed of light (non-relativistic speed refers to speeds too slow to qualify). Ultrarelativistic speed is a speed close to the speed of light. For example, 1/3 the speed of light could be considered relativistic and 9/10 the speed of light ultrarelativistic. 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.