Noah Webster's New International Dictionary of the English Language
1. (v. i.) To slip or slide back, to suffer a return of a disease.
2. (v. i.) 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.
3. (v. i.) To fall from Christian faith into paganism, heresy, or unbelief; to backslide.
4. (n.) A sliding or falling back, especially into a former bad state, either of body or morals; backsliding; the state of having fallen back.
5. (n.) One who has relapsed, or fallen back, into error; a backslider; specifically, one who, after recanting error, returns to it again.