The Collaborative International Dictionary of English v.0.48:

Fortitude \For"ti*tude\, n. [L. fortitudo, fr. fortis strong.
   See Fort.]
   1. Power to resist attack; strength; firmness. [Obs.]
      [1913 Webster]

            The fortitude of the place is best known to you.
                                                  --Shak.
      [1913 Webster]

   2. That strength or firmness of mind which enables a person
      to encounter danger with coolness and courage, or to bear
      pain or adversity without murmuring, depression, or
      despondency; passive courage; resolute endurance; firmness
      in confronting or bearing up against danger or enduring
      trouble.
      [1913 Webster]

            Extolling patience as the truest fortitude.
                                                  --Milton.
      [1913 Webster]

            Fortitude is the guard and support of the other
            virtues.                              --Locke.

   Syn: Courage; resolution; resoluteness; endurance; bravery.
        See Courage, and Heroism.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fortitude
    n 1: strength of mind that enables one to endure adversity with
         courage