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.
      [1913 Webster]

   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.
      [1913 Webster]

            That task performed, [preachers] relapse into
            themselves.                           --Cowper.
      [1913 Webster]

   3. (Theol.) To fall from Christian faith into paganism,
      heresy, or unbelief; to backslide.
      [1913 Webster]

            They enter into the justified state, and so continue
            all along, unless they relapse.       --Waterland.
      [1913 Webster]

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.
      [1913 Webster]

            Alas! from what high hope to what relapse
            Unlooked for are we fallen!           --Milton.
      [1913 Webster]

   2. One who has relapsed, or fallen back, into error; a
      backslider; specifically, one who, after recanting error,
      returns to it again. [Obs.]
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "relapse":
   about-face, apostasy, backing, backset, backslide, backsliding,
   backward motion, backward step, cock, decline, degenerate,
   degeneration, deteriorate, deterioration, disenchantment,
   disimprove, fade, fading, fail, failing, failure, fall,
   fall again into, fall astern, fall back, fall behind,
   fall from grace, falling back, flip-flop, get back, get behind,
   get worse, go back, go backwards, go behind, grow worse,
   have a relapse, jerk back, lapse, lapse back, lapsing, let down,
   lose ground, pull back, reaction, recede, recession, recidivate,
   recidivation, recidivism, reclamation, reconversion, recur to,
   reentry, refluence, reflux, regress, regressing, regression,
   rehabilitation, reinstatement, restitution, restoration, retreat,
   retroaction, retrocede, retrocession, retroflex, retroflexion,
   retrogradation, retrograde, retrogress, retrogression,
   retroversion, retrovert, retrusion, return, return to, returning,
   reversal, reverse, reversion, revert, revert to, reverting,
   revulsion, rollback, setback, sicken, sink, sink back, sinking,
   slacken, slide back, slip back, slipping back, sternway, throwback,
   turn, turnabout, weaken, weakening, worsen, worsening,
   yield again to



Bouvier's Law Dictionary, Revised 6th Ed (1856):

RELAPSE. The condition of one who, after having abandoned a course of vice,
returns to it again. Vide Recidive.