Cold launch, applied to guided missiles, means that the main, high-temperature rocket motor does not fire until the missile is outside its launcher. For shoulder-fired infantry missiles such as the FGM-148 Javelin or FIM-92 Stinger, rocket ignition needs to be delayed long enough so it is of no danger to the crew, and there is no backblast if it is fired from a closed room.
The technique is a practical necessity for submarine-launched ballistic missiles, where the hot exhaust of a rocket motor could be catastrophic.
Vertical launch systems on surface warships vary; the U.S. prefers hot launch while the Russians use cold launch. VLS designers argue both ways; the cold launch method needs more extensive piping for the ejection system and a more complex firing system, while the hot launch system requires the ship to be able to withstand and vent the hot gas.
Energy for the cold launch ejection comes from a compressed gas cylinder, or a pyrotechnic gas generator whose exhaust is of low temperature.