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

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "fortitude":
   acceptance, backbone, boldness, bottom, bravery, bulldog courage,
   cardinal virtues, charity, constancy, courage, courageousness,
   dauntlessness, decisiveness, determination, endurance, energy,
   faith, fearlessness, fixity of purpose, forbearance, forbearing,
   forbearingness, force, gameness, gaminess, grit, grittiness, guts,
   gutsiness, hardihood, hardiness, heartiness, hope, indulgence,
   intestinal fortitude, intrepidity, iron will, justice, lenience,
   leniency, long-sufferance, long-suffering, longanimity, love,
   lustihood, lustiness, mettle, mettlesomeness, might, mightiness,
   moral courage, moral fiber, natural virtues, nerve, nerviness,
   obstinacy, patience, patience of Job, patientness, perseverance,
   pertinaciousness, pertinacity, pith, pluckiness, potency, power,
   powerfulness, prudence, resoluteness, resolution, robustness,
   ruggedness, sand, self-control, spirit, spunk, spunkiness,
   stalwartness, stamina, staying power, stick-to-itiveness,
   sticking power, stoicism, stoutness, strength, strength of mind,
   strength of purpose, strength of will, sturdiness, sufferance,
   supernatural virtues, sweet reasonableness, temperance,
   tenaciousness, tenacity, theological virtues, tolerance,
   toleration, toughness, valiancy, valor, valorousness, vigor,
   vigorousness, vitality, waiting game, waiting it out, will,
   will of iron, will power