That strength of mind or courage which enables a person to bear up calmly under opposition, adversity, or affliction.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (noun) (n.) (noun)
Strength to endure suffering bravely.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)
That power of mind which enables a person to act or suffer with patience, and without flinching or complaining; endurance; resolution.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (noun) (n.) (noun)
That strength of mind which enables one to meet danger or endure pain with calmness.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (noun) (n.) (noun)
Firmness of mind; resolute endurance.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (noun) (n.) (noun)

Usage examples:

The heroic man still preserved his fortitude
- Illustrative Anecdotes of the Animal Kingdom by Samuel Griswold Goodrich
It was a time for fortitude Jeanne would have neither kisses, nor words, nor tears.
- The Saint by Antonio Fogazzaro Commentator: William Roscoe Thayer
With woman's faith in that divine being who alone could avail them, and with woman's high mental fortitude in moments of protracted trial, they had both known how to control the exhibition of their terrors, and had sought their support in the same appeal to a power superior to all of earth.
- The Water-Witch or, The Skimmer of the Seas by James Fenimore Cooper