Definition of "fortitude"

I am able now (me thinkes) / (Out of a Fortitude of Soule, I feele) / To endure more Miſeries, […]

1613 (date written), William Shakespeare, [John Fletcher], “The Famous History of the Life of King Henry the Eight”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act III, scene ii], page 223, column 1

I shall soon have need for all my fortitude, as I am on the point of separation from my own daughter.

c. 1794, Jane Austen, “[Lady Susan.] I. Lady Susan Vernon to Mr. Vernon.”, in J[ames] E[dward] Austen[-]Leigh, A Memoir of Jane Austen: […] to which is Added Lady Susan and Fragments of Two Other Unfinished Tales by Miss Austen, 2nd edition, London: Richard Bentley and Son, […], published 1871, pages 203–204

She [the ship] may be saved by your efforts, by your resource and fortitude bearing up against the heavy weight of guilt and failure.

1904–1906, Joseph Conrad, “The Grip of the Land”, in The Mirror of the Sea, 1st American edition, New York, N.Y., London: Harper & Brothers, published October 1906, page 123

Mitt Romney […] charges that [Barack] Obama is an appeaser who apologizes for America, lacks fortitude and is "tentative, indecisive, timid and nuanced."

2012 January 30, Fareed Zakaria, “The Strategist”, in Time‎, archived from the original on 26 July 2012

The proper fans might also expend time—about twenty hours—trying to visit every Tube station as quickly as possible. […] But to visit every station in record time—which people have been attempting since at least 1960—requires real fortitude. You must get up before dawn, you must drink or eat almost nothing, so niggardly is the Underground with toilet provision.

2012, Andrew Martin, Underground Overground: A Passenger’s History of the Tube, Profile Books, page 267

The Turke with moſt mighty preparation makes for Cipres. Othello, the fortitude of the place, is beſt knowne to you, […]

c. 1603–1604 (date written), William Shakespeare, The Tragœdy of Othello, the Moore of Venice. […] (First Quarto), London: […] N[icholas] O[kes] for Thomas Walkley, […], published 1622, [Act I, scene iii], page 15

Hey day! VVhat are all the VVomen of my Family abroad? Is not my Wife come home? Nor my Siſter, nor my Daughter? […] Mercy on us, what can be the meaning of it? Sure the Moon is in all her Fortitudes; […]

1695, [William] Congreve, Love for Love: A Comedy. […], London: […] Jacob Tonson, […], Act II, scene i, page 17