The Collaborative International Dictionary of English v.0.48:

Escape \Es*cape"\, n.
   1. The act of fleeing from danger, of evading harm, or of
      avoiding notice; deliverance from injury or any evil;
      flight; as, an escape in battle; a narrow escape; also,
      the means of escape; as, a fire escape.
      [1913 Webster]

            I would hasten my escape from the windy storm. --Ps.
                                                  lv. 8.
      [1913 Webster]

   2. That which escapes attention or restraint; a mistake; an
      oversight; also, transgression. [Obs.]
      [1913 Webster]

            I should have been more accurate, and corrected all
            those former escapes.                 --Burton.
      [1913 Webster]

   3. A sally. "Thousand escapes of wit." --Shak.
      [1913 Webster]

   4. (Law) The unlawful permission, by a jailer or other
      custodian, of a prisoner's departure from custody.
      [1913 Webster]

   5. (Bot.) A plant which has escaped from cultivation.
      [Webster 1913 Suppl.]

   Note: Escape is technically distinguishable from prison
         breach, which is the unlawful departure of the prisoner
         from custody, escape being the permission of the
         departure by the custodian, either by connivance or
         negligence. The term escape, however, is applied by
         some of the old authorities to a departure from custody
         by stratagem, or without force. --Wharton.
         [1913 Webster]

   5. (Arch.) An apophyge.
      [1913 Webster]

   6. Leakage or outflow, as of steam or a liquid.
      [1913 Webster]

   7. (Elec.) Leakage or loss of currents from the conducting
      wires, caused by defective insulation.
      [1913 Webster]

   Escape pipe (Steam Boilers), a pipe for carrying away steam
      that escapes through a safety valve.

   Escape valve (Steam Engine), a relief valve; a safety
      valve. See under Relief, and Safety.

   Escape wheel (Horol.), the wheel of an escapement.
      [1913 Webster]

WordNet (r) 3.0 (2006):

escape valve
    n 1: a valve in a container in which pressure can build up (as a
         steam boiler); it opens automatically when the pressure
         reaches a dangerous level [syn: safety valve, relief
         valve, escape valve, escape cock, escape]