observant

adjective

1. quick to notice or perceive; alert.
2. looking at, watching, or regarding attentively; watchful.
3. careful in the observing of a law, custom, religious ritual, or the like.

noun

4. an observer of law or rule.
5. (initial capital letter) Also Ob·ser·van·tine [ob-zur-vuhn-tin, -teen] /ɒbˈzɜr vən tɪn, -ˌtin/. a member of a Franciscan order that observes the strict rule of St. Francis.

adjective

1. paying close attention to detail; watchful or heedful
2. adhering strictly to rituals, ceremonies, laws, etc

adj.1590s, from observe + -ant, or else from French observant, past participle of observer (see observance). In reference to Judaism, from 1902. As a noun from late 15c. Related: Observantly; observantness.