An Etymological Dictionary of Astronomy and Astrophysics

1. Paying strict attention.
   

2. Careful in observing (Merriam-Webster.com).

Etymology (EN): → observe + -ant, a suffix forming adjective, → -ance.

Etymology (PE): Nepâhdâr, from nepâh present stem of nepâhidan, → observe, + dâr preset stem and agent noun of dâštan, dâridan “to have, hold, maintain, possess,” → protected band.