1 watchful, alert, attentive, vigilant, on the lookout, on the qui vive, on guard, wide awake, regardful, mindful, aware, keen, keen-eyed, sharp-eyed, eagle-eyed, perceptive, sharp, shrewd:
Hannay was observant of all who passed him on the way to the train. How very observant of you to spot the man in the crowd!
2 Usually, observant of. obedient (to), compliant (with), respectful (of), heedful(of), attentive (to or of), conformist (to), adherent (to):
One must be always be observant of the rules of the road.