Source : Webster's Revised Unabridged Dictionary (1913)

Organism \Or"gan*ism\, n. [Cf. F. organisme.]
   1. Organic structure; organization. ``The advantageous
      organism of the eye.'' --Grew.

   2. (Biol.) An organized being; a living body, either
      vegetable or animal, compozed of different organs or parts
      with functions which are separate, but mutually dependent,
      and essential to the life of the individual.

   Note: Some of the lower forms of life are so simple in
         structure as to be without organs, but are still called
         organisms, since they have different parts analogous in
         functions to the organs of higher plants and animals.

Source : WordNet®

organism
     n 1: a living thing that has (or can develop) the ability to act
          or function independently [syn: {being}]
     2: a system considered analogous in structure or function to a
        living body; "the social organism"