The Collaborative International Dictionary of English v.0.48:

Relapse \Re*lapse"\ (r?-l?ps"), v. i. [imp. & p. p. Relapsed
   (-l?pst"); p. pr. & vb. n. Relapsing.] [L. relapsus, p. p.
   of relabi to slip back, to relapse; pref. re- re- + labi to
   fall, slip, slide. See Lapse.]
   1. To slip or slide back, in a literal sense; to turn back.
      [Obs.] --Dryden.
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   2. To slide or turn back into a former state or practice; to
      fall back from some condition attained; -- generally in a
      bad sense, as from a state of convalescence or amended
      condition; as, to relapse into a stupor, into vice, or
      into barbarism; -- sometimes in a good sense; as, to
      relapse into slumber after being disturbed.
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            That task performed, [preachers] relapse into
            themselves.                           --Cowper.
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   3. (Theol.) To fall from Christian faith into paganism,
      heresy, or unbelief; to backslide.
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            They enter into the justified state, and so continue
            all along, unless they relapse.       --Waterland.
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The Collaborative International Dictionary of English v.0.48:

Relapse \Re*lapse"\, n. [For sense 2 cf. F. relaps. See
   Relapse, v.]
   1. A sliding or falling back, especially into a former bad
      state, either of body or morals; backsliding; the state of
      having fallen back.
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            Alas! from what high hope to what relapse
            Unlooked for are we fallen!           --Milton.
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   2. One who has relapsed, or fallen back, into error; a
      backslider; specifically, one who, after recanting error,
      returns to it again. [Obs.]
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WordNet (r) 3.0 (2006):

relapse
    n 1: a failure to maintain a higher state [syn: backsliding,
         lapse, lapsing, relapse, relapsing, reversion,
         reverting]
    v 1: deteriorate in health; "he relapsed" [syn: get worse,
         relapse] [ant: bounce back, get over, get well]
    2: go back to bad behavior; "Those who recidivate are often
       minor criminals" [syn: relapse, lapse, recidivate,
       regress, retrogress, fall back]