What does "jihad" mean?

Jihad: A religious war against infidels or Mohammedan heretics; also, any bitter war or crusade for a principle or belief. [Their] courage in war . . . had not, like that of the Mohammedan dervishes of the Sudan, or of Mohammedans anywhere engaged in a jehad, a religious motive and the promise of future bliss behind it. James Bryce.