Sublight, also called subwarp, space-normal speed, or sub-warp speed, was a term that referred to anything below c, the speed of light, whose value is exactly 299,792,458 meters per second.
Sublight was usually used to describe non-warp speeds, typically when either an impulse drive or thrusters would be used as propulsion for a vessel.
Sublight speeds were measured in sublight factors. Alternatively, warp factors, expressed as lower fractional values, were used to measure sublight speeds. (TOS: "Elaan of Troyius", Star Trek: The Motion Picture)
In 2018, scientific advancements in sublight propulsion made sleeper ships obsolete. (TOS: "Space Seed")
Captain James T. Kirk ordered the USS Enterprise to head for Makus III at sublight speed, fearing he might never see the Galileo crew again. (TOS: "The Galileo Seven")
Bajoran lightships traveled at sublight speeds. (DS9: "Explorers")