The Collaborative International Dictionary of English v.0.48:

Organism \Or"gan*ism\, n. [Cf. F. organisme.]
   1. Organic structure; organization. "The advantageous
      organism of the eye." --Grew.
      [1913 Webster]

   2. (Biol.) An organized being; a living body, either
      vegetable or animal, composed of different organs or parts
      with functions which are separate, but mutually dependent,
      and essential to the life of the individual.
      [1913 Webster]

   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.
         [1913 Webster] organismal

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "organism":
   aerobic organism, an existence, anaerobic organism, anatomy,
   architectonics, architecture, arrangement, autotrophic organism,
   being, body, build, building, composition, conformation,
   constitution, construction, creation, creature, critter, entelechy,
   entity, fabric, fabrication, fashion, fashioning, forging, form,
   format, formation, frame, genetic individual, getup,
   heterotrophic organism, individual, life, living being,
   living thing, make, makeup, making, manufacture, microbe,
   microorganism, mold, molding, monad, morphological individual,
   object, ont, organic being, organic structure, organization,
   pattern, patterning, person, persona, personality,
   physiological individual, physique, plan, production, setup, shape,
   shaping, something, soul, structure, structuring, tectonics,
   texture, thing, tissue, unit, virus, warp and woof, weave, web,
   zooid, zoon