Sublight (or space-normal speed) is a term that refers to anything below the speed of light, approximately 3 × 10E8 meters/second.
Sublight speed is usually used to describe non-warp velocities -- typically when either an impulse drive or thrusters would be used as propulsion for a vessel -- though an impulse-powered flight of half lightspeed may be described as "Warp point 5". (Star Trek: The Motion Picture)
As far as it is known, Earth spacecraft had no means of propelling ships to the speed of light or greater until Zefram Cochrane's Phoenix test-flight in 2063. (Star Trek: First Contact) Impulse drive, developed c. 2018 (TOS: "Space Seed"), initially drove spacecraft to 0.25 c. (TOS novel: The Rings of Time) This made exploration and colonization within the Sol system feasible, less so interstellar exploration and colonization -- though many generation ships of the 21st and even early 22nd centuries undertook decades-long journeys to do so. (TAS: "The Terratin Incident", Star Trek Spaceflight Chronology, Star Trek: Shadowstar Station: "Last Night I Dreamed Somebody Loved Me", "Esteban")
All sublight vessels were subject to time-dilatation effects, the moreso as they approached light speed.
- Warp drive may have alleviated, though not eliminated, time dilatation. (Star Trek: Avenger: "Belonging") If so, that may have been the impetus for developing the stardate system.
In addition to impulse (essentially a nuclear-fusion drive), thruster and ion drives, other means of sublight propulsion included the use of solar sails and even the use of nuclear-fission bombs against a ship's pusher plate. Many starfaring civilizations experimented with this latter method; on Earth, it was widely known as Project Orion. Another system, using nuclear fission and chemical propellants, was known as NERVA, for Nuclear Engine for Rocket Vehicle Application.
- DY-100 class vessels, such as the SS Botany Bay, seemed to be NERVA-driven ships, but this has not been stated in canon. Some fanon may have directly stated this.