The Collaborative International Dictionary of English v.0.48:

Relief \Re*lief"\ (r?-l?f"), n. [OE. relef, F. relief, properly,
   a lifting up, a standing out. See Relieve, and cf.
   Basrelief, Rilievi.]
   1. The act of relieving, or the state of being relieved; the
      removal, or partial removal, of any evil, or of anything
      oppressive or burdensome, by which some ease is obtained;
      succor; alleviation; comfort; ease; redress.
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            He sees the dire contagion spread so fast,
            That, where it seizes, all relief is vain. --Dryden.
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   2. Release from a post, or from the performance of duty, by
      the intervention of others, by discharge, or by relay; as,
      a relief of a sentry.
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            For this relief much thanks; 'tis bitter cold.
                                                  --Shak.
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   3. That which removes or lessens evil, pain, discomfort,
      uneasiness, etc.; that which gives succor, aid, or
      comfort; also, the person who relieves from performance of
      duty by taking the place of another; a relay.
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   4. (Feudal Law) A fine or composition which the heir of a
      deceased tenant paid to the lord for the privilege of
      taking up the estate, which, on strict feudal principles,
      had lapsed or fallen to the lord on the death of the
      tenant.
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   5. (Sculp. & Arch.) The projection of a figure above the
      ground or plane on which it is formed.
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   Note: Relief is of three kinds, namely, high relief
         (altorilievo), low relief, (basso-rilievo), and
         demirelief (mezzo-rilievo). See these terms in the
         Vocabulary.
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   6. (Paint.) The appearance of projection given by shading,
      shadow, etc., to any figure.
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   7. (Fort.) The height to which works are raised above the
      bottom of the ditch. --Wilhelm.
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   8. (Physical Geog.) The elevations and surface undulations of
      a country. --Guyot.
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   Relief valve, a valve arranged for relieving pressure of
      steam, gas, or liquid; an escape valve.
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   Syn: Alleviation; mitigation; aid; help; succor; assistance;
        remedy; redress; indemnification.
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WordNet (r) 3.0 (2006):

relief 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]