Source : Webster's Revised Unabridged Dictionary (1913)

Involuntary \In*vol"un*ta*ry\, a. [L. involuntarius. See {In-}
   not, and {Voluntary}.]
   1. Not having will or the power of choice.

   2. Not under the influence or control of the will; not
      voluntary; as, the involuntary movements of the body;
      involuntary muscle fibers.

   3. Not proceeding from choice; done unwillingly; reluctant;
      compulsory; as, involuntary submission.

Source : WordNet®

involuntary
     adj 1: not subject to the control of the will; "involuntary
            manslaughter"; "involuntary servitude"; "an
            involuntary shudder"; "It (becoming a hero) was
            involuntary. They sank my boat"- John F.Kennedy [syn:
            {nonvoluntary}, {unvoluntary}] [ant: {voluntary}]
     2: controlled by the autonomic nervous system; without
        conscious control; "involuntary muscles"; "gave an
        involuntary start" [ant: {voluntary}]