Sublight or space-normal 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.
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")