In the year 622 A.D., persecution against Muhammad and his followers became especially brutal. Muhammad had received a number of threats against his life, including attempted assassinations. In order to protect both himself and his people, Muhammad sent his followers, which consisted of about 60 separate families, to the town of Yathrib. Muhammad himself traveled to Yathrib secretly shortly thereafter.
Followers of Islam, known as Muslims, call this the Hijrah. Hijrah means ‘great emigration.’ The year that the Hijrah took place marks the first year of the Islamic calendar, and is the year that most historians consider the beginning of the Islamic era.