The Collaborative International Dictionary of English v.0.48:

Jealous \Jeal"ous\, a. [OE. jalous, gelus, OF. jalous, F.
   jaloux, LL. zelosus zealous, fr. zelus emulation, zeal,
   jealousy, Gr. zh^los. See Zeal, and cf. Zealous.]
   [1913 Webster]
   1. Zealous; solicitous; vigilant; anxiously watchful.
      [1913 Webster]

            I have been very jealous for the Lord God of hosts.
                                                  --Kings xix.
                                                  10.
      [1913 Webster]

            How nicely jealous is every one of us of his own
            repute!                               --Dr. H. More.
      [1913 Webster]

   2. Apprehensive; anxious; suspiciously watchful.
      [1913 Webster]

            'This doing wrong creates such doubts as these,
            Renders us jealous and disturbs our peace. --Waller.
      [1913 Webster]

            The people are so jealous of the clergy's ambition.
                                                  --Swift.
      [1913 Webster]

   3. Demanding exclusive devotion; intolerant of rivalry.
      [1913 Webster]

            Thou shalt worship no other God; for the Lord, whose
            name is Jealous, is a jealous God.    --Ex. xxxiv.
                                                  14.
      [1913 Webster]

   4. Disposed to suspect rivalry in matters of interest and
      affection; apprehensive regarding the motives of possible
      rivals, or the fidelity of friends; distrustful; having
      morbid fear of rivalry in love or preference given to
      another; painfully suspicious of the faithfulness of
      husband, wife, or lover.
      [1913 Webster]

            If the spirit of jealousy come upon him, and he be
            jealous of his wife.                  --Num. v. 14.
      [1913 Webster]

            To both these sisters have I sworn my love:
            Each jealous of the other, as the stung
            Are of the adder.                     --Shak.
      [1913 Webster]

            It is one of the best bonds, both of chastity and
            obedience, in the wife, if she think her husband
            wise; which she will never do if she find him
            jealous.                              --Bacon.

   Syn: Suspicious; anxious; envious.

   Usage: Jealous, Suspicious. Suspicious is the wider term.
          We suspect a person when we distrust his honesty and
          imagine he has some bad design. We are jealous when we
          suspect him of aiming to deprive us of what we dearly
          prize. Iago began by awakening the suspicions of
          Othello, and converted them at last into jealousy.
          "Suspicion may be excited by some kind of accusation,
          not supported by evidence sufficient for conviction,
          but sufficient to trouble the repose of confidence."
          "Jealousy is a painful apprehension of rivalship in
          cases that are peculiarly interesting to us." --Cogan.
          [1913 Webster]

WordNet (r) 3.0 (2006):

jealous
    adj 1: showing extreme cupidity; painfully desirous of another's
           advantages; "he was never covetous before he met her";
           "jealous of his success and covetous of his possessions";
           "envious of their art collection" [syn: covetous,
           envious, jealous]
    2: suspicious or unduly suspicious or fearful of being displaced
       by a rival; "a jealous lover" [syn: jealous, green-eyed,
       overjealous]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "jealous":
   anxious, begrudging, bitter, covetous, demanding, desirous of,
   distrustful, doubting, envious, enviousness, envying, grasping,
   green, green with envy, green with jealousy, green-eyed, grudging,
   horn-mad, insecure, invidious, invidiousness, jaundice-eyed,
   jaundiced, jealousy, mistrustful, mistrusting, possessive,
   possessory, questioning, resentful, suspicious, threatened,
   vulnerable, yellow, yellow-eyed



The Devil's Dictionary (1881-1906):

JEALOUS, adj.  Unduly concerned about the preservation of that which
can be lost only if not worth keeping.