The Collaborative International Dictionary of English v.0.48:

Plastid \Plas"tid\, Plastide \Plas"tide\, n. [Gr. ?, ?, a
   creator.]
   1. (Biol.) A formative particle of albuminous matter; a
      monad; a cytode. See the Note under Morphon. [Archaic]
      --Haeckel.
      [1913 Webster]

   2. (Bot.) any of several types of minute granules found in
      the protoplasm of vegetable cells, having their own
      membrane, robosomes, and DNA. Among plant cells the most
      common are chloroplasts, which contain the chlorophyll
      and the photosynthetic machinery of the cell. They are
      divided by their colors into three classes,
      chloroplastids, chromoplastids, and leucoplastids.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

plastid
    n 1: any of various small particles in the cytoplasm of the
         cells of plants and some animals containing pigments or
         starch or oil or protein