A cold start is a highly dangerous procedure used only in an emergency situation that necessitates the reinitialization of a deactivated or "cold" antimatter reactor in a very short period of time rather than the usual minimum requirement of thirty minutes.
One of the first recorded instances of such a startup procedure, which had previously been only theoretical, occurred in 2266 and was performed by Chief Engineer Montgomery Scott and First Officer Spock to prevent the destruction of the USS Enterprise. (TOS: "The Naked Time")
Around Stardate 9711 the USS Valkyrie (NCC-2590) used the cold start procedure to escape the destruction of an alien power generation system equivalent to "a hundred warp cores". Despite the confidence of Chief Engineer Elberg, some crew members were unsure if the technique was even possible. (Star Trek: Valkyrie: "The Nebula")