The Collaborative International Dictionary of English v.0.48:

Chlorophyll \Chlo"ro*phyll\, n. [Gr. chlwro`s light green +
   fy`llon leaf: cf. F. chlorophylle.]
   1. (Bot.) Literally, leaf green; a green granular matter
      formed in the cells of the leaves (and other parts exposed
      to light) of plants, to which they owe their green color,
      and through which all ordinary assimilation of plant food
      takes place. Similar chlorophyll granules have been found
      in the tissues of the lower animals. [Written also
      chlorophyl.]
      [1913 Webster]

   2. any of a group of green pigments found in photosynthetic
      organisms. Chlorophyll a and chlorophyll b are found
      in higher plants and green algae; chlorophyll c is found
      in certain types of marine algae. Chemically, it has a
      porphyrin ring with a magnesium ion bound to the four
      central nitrogens, and has a phytyl side chain. It is
      essential for photosynthesis in most plants. Chlorophyll
      a has formula C55H72N4O5Mg.

   Syn: chlorophyll.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

chlorophyll b
    n 1: a dark-green plant pigment having a brilliant green alcohol
         solution; generally characteristic of higher plants