Source : Webster's Revised Unabridged Dictionary (1913)

Hegira \He*gi"ra\ (?; 277), n. [Written also {hejira}.] [Ar.
   hijrah flight.]
   The flight of Mohammed from Mecca, September 13, A. D. 622
   (subsequently established as the first year of the Moslem
   era); hence, any flight or exodus regarded as like that of
   Mohammed.

   Note: The starting point of the Era was made to begin, not
         from the date of the flight, but from the first day of
         the Arabic year, which corresponds to July 16, A. D.
         622.

Source : WordNet®

Hegira
     n 1: the flight of Muhammad from Mecca to Medina in 622 which
          marked the beginning of the Muslim era; the Muslim
          calendar begins in that year [syn: {Hejira}]
     2: a journey by a large group to escape from a hostile
        environment [syn: {exodus}, {hejira}]