The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hejira \He*ji"ra\, n.
   See Hegira. Hektoliter
   Hektogram
   Hektare

WordNet (r) 3.0 (2006):

Hejira
    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: Hegira, Hejira]
    2: a journey by a large group to escape from a hostile
       environment [syn: exodus, hegira, hejira]