The Collaborative International Dictionary of English v.0.48:

organelle \organelle\ n. (Biol.)
   a specialized part of a cell performing a specific function,
   usually visible under the microscope as a distinct object; it
   is analogous to an organ[2], but on a microscopic scale.

   Syn: cell organ.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

organelle
    n 1: a specialized part of a cell; analogous to an organ; "the
         first organelle to be identified was the nucleus" [syn:
         organelle, cell organelle, cell organ]