An emergency thruster was a type of thruster.
In 2286, the USS Saratoga was approached by the Whale Probe. After the probe had transmitted an amplification wave of immense power at the starship, the ship's science officer reported that the call was affecting all systems. Fearing a collision with the probe, the captain ordered the helmsman to reduce the closing speed by using thrusters. After he reported that the thruster controls had been neutralized, the captain ordered emergency thrusters. The helmsman attempted unsuccessfully to fire the thrusters. (Star Trek IV: The Voyage Home)