Definition of Anno hegirae

The Islamic calendar or Muslim calendar (Arabic: التقويم الهجري; at-taqwīm al-hijrī; Persian: تقویم هجري قمری ‎ taqwīm-e hejri-ye qamari; also called the Hijri calendar) is the calendar used to date events in many predominantly Muslim countries, and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days. It is a lunar calendar having 12 lunar months in a year of about 354 days. Because this lunar year is about 11 days shorter than the solar year, Islamic holy days, although celebrated on fixed dates in their own calendar, usually shift 11 days earlier each successive solar year, such as a year of the Gregorian calendar. Islamic years are also called Hijra years because the first year was the year during which the Hijra occurred—  Muhammad's emigration from Mecca to Medina. Thus each numbered year is designated either H or AH, the latter being the initials of the Latin anno Hegirae (in the year of the Hijra).