observant

Short definition from Wiktionary

Adjective

1. 1
   Alert and paying close attention; watchful.
2. 2
   Diligently attentive in observing a law, custom, duty or principle; regardful; mindful.
3. 3
   Practicing a religion.

Noun

1. 1
   A member of a Franciscan order that strictly observes the rules of St. Francis.