1. paying close attention especially to details

Similar word(s): attentive

2. quick to notice; showing quick and keen perception

Similar word(s): perceptive, observing

3. (of individuals) adhering strictly to laws and rules and customs

- observant of the speed limit

Similar word(s): lawful

Sentences with observant as an adjective:

- The observant police officer noticed that my tax disk was out-of-date.

- I was normally observant of the local parking restrictions.