A term for an object travelling close to light speed (but still below it). This is generally defined as anywhere from 10% light speed up to (but not including) light speed. At these speeds, the effects of Einstein's Theory of Relativity start to become noticeable, meaning objects travelling this fast have much higher masses and energies. A pebble thrown at relativistic speed could impact with the force of a nuke or more. Often the term "sublight speed" is used instead, but this is misleading as "sublight speed" could mean any speed slower than lightspeed. Time dilation also starts to become noticeable, as explained here by Carl Sagan:

https://www.youtube.com/watch?v=Uy7rrrCQh2w

Many fictional entities can travel at relativistic speed without actually manifesting relativistic effects.

The equation for calculating relativistic kinetic energy is mc^2/the square root of (v^2/c^2), where m = the object's mass in kilograms, c = the speed of light in meters/second (around 299,792,458), and v = the object's velocity in meters/second.

See also:

- FTL

- Hypersonic

- Massively Hypersonic

- Omnipresent

- Speed

- Speed of Light

- Speed of Sound

- Sub-Relativistic

- Supersonic