The Collaborative International Dictionary of English v.0.48:

Observant \Ob*serv"ant\, a. [L. observans, -anits, p. pr. of
   observare: cf. F. observant. See Observe.]
   1. Taking notice; viewing or noticing attentively; watchful;
      carefully attentive; as, an observant spectator; observant
      habits.
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            Wandering from clime to clime observant stray'd.
                                                  --Pope.
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   2. Submissively attentive; obediently watchful; regardful;
      mindful; obedient (to); -- with of, as, to be observant of
      rules.
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            We are told how observant Alexander was of his
            master Aristotle.                     --Sir K.
                                                  Digby.
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The Collaborative International Dictionary of English v.0.48:

Observant \Ob*serv"ant\, n.
   1. One who observes forms and rules. [Obs.] --Hooker.
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   2. A sycophantic servant. [Obs.]
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            Silly ducking observants,
            That stretch their duties nicely.     --Shak.
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   3. (R. C. Ch.) An Observantine.
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WordNet (r) 3.0 (2006):

observant
    adj 1: paying close attention especially to details
    2: quick to notice; showing quick and keen perception [syn:
       observant, observing]
    3: (of individuals) adhering strictly to laws and rules and
       customs; "law-abiding citizens"; "observant of the speed
       limit" [syn: law-abiding, observant]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "observant":
   Argus-eyed, active, advertent, agog, alert, all ears, all eyes,
   assiduous, attentive, aux aguets, awake, aware, careful, compliant,
   concentrated, conforming, conscientious, conscious, constant,
   custodial, deferential, devoted, devout, diligent, duteous,
   dutiful, eagle-eyed, earnest, ethical, faithful, finical,
   finicking, finicky, guarded, hawk-eyed, heedful, intense, intent,
   intentive, keen, keen-eyed, lidless, loyal, meticulous, mindful,
   moral, nice, niggling, obedient, observing, on guard, on the ball,
   on the job, on the lookout, on the watch, open-eared, open-eyed,
   openmouthed, perceptive, practicing, prudent, punctilious,
   punctual, regardful, respectful, scrupulous, sharp, sharp-eyed,
   shrewd, sleepless, thoughtful, true, vigilant, wary, watchful,
   wide awake, with open eyes