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.)