Source : Webster's Revised Unabridged Dictionary (1913)

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;
      attentive; as, an observant spectator; observant habits.

            Wandering from clime to clime observant stray'd.
                                                  --Pope.

   2. Submissively attentive; obediently watchful; regardful;
      mindful; obedient (to); -- with of, as, to be observant of
      rules.

            We are told how observant Alexander was of his
            master Aristotle.                     --Sir K.
                                                  Digby.

Observant \Ob*serv"ant\, n.
   1. One who observes forms and rules. [Obs.] --Hooker.

   2. A sycophantic servant. [Obs.]

            Silly ducking observants, That stretch their duties
            nicely.                               --Shak.

   3. (R.C.Ch.) An Observantine.

Source : WordNet®

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