The emergency hatch was a hatch found on the bridges of starships that was used to escape in the case of an emergency.
The crew of the USS Enterprise used the escape hatch on the bridge of the HMS Bounty after the Klingon Bird-of-Prey crashed into San Francisco Bay in 2286. (Star Trek IV: The Voyage Home)
While under the influence of the polywater intoxication, the bridge crew of the SS Tsiolkovsky released the emergency hatch of their ship, subsequently blowing them out into space, and to their deaths. (TNG: "The Naked Now")
After the USS Yangtzee Kiang crash landed on the surface of a penal moon located 0.35 light years from the Idran system terminus. (DS9: "Battle Lines")