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:

Relapsing \Re*laps"ing\, a.
   Marked by a relapse; falling back; tending to return to a
   former worse state.
   [1913 Webster]

   Relapsing fever (Med.), an acute, epidemic, contagious
      fever, which prevails also endemically in Ireland, Russia,
      and some other regions. It is marked by one or two
      remissions of the fever, by articular and muscular pains,
      and by the presence, during the paroxism of spiral
      bacterium (Spirochaete) in the blood. It is not usually
      fatal. Called also famine fever, and recurring fever.
      [1913 Webster]

WordNet (r) 3.0 (2006):

relapsing
    n 1: a failure to maintain a higher state [syn: backsliding,
         lapse, lapsing, relapse, relapsing, reversion,
         reverting]