It follows the Hijri calendar (5)
I believe the answer is:
islam
'it follows the hijri calendar' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for islam that I've seen before include "Major world religion" , "Religion based on Muhammad's teachings" , "Religion based on teachings of Muhammad" , "Religious faith of Muslims" , "Mails to great eastern religion" .)