Carte blanche is French for white paper (though blanche here more accurately means blank, and carte is versatile in French, allowing for several alternative translations such as blank map and blank ticket). In English, we use it as a figurative noun meaning total freedom to act or full discretionary power. A person with carte blanche is free to fill the figurative blank paper with his or her own rules and plans.
The loan phrase came to English in the 18th century. It caught on quickly, filling a gap in the language—it has few synonyms, though blank check sometimes comes close—and has been used consistently ever since.
The French plural is cartes blanches, but this rarely comes up in English because we usually treat carte blanche as a mass noun, meaning it takes no article and can’t be plural; we write “I have carte blanche” and “they were given carte blanche,” not “I have a carte blanche” and “they were given cartes blanches.”
The phrase was italicized in early use, but like most long-established loanwords and phrases, it now goes unitalicized.
[T]he ministers gave him an Aga to tie up his papers, appointed him master of his own revels, and presented him with a carte blanche as to the dress of the army. [Gulzara, princess of Persia (1816)]
Lord Auckland, we are told, has had carte blanche from the Home government to act as he thinks fit with regard to China. [Campaign of the Indus, a series of letters … (1840)]
It is the custom that the great and powerful have carte blanche to seize people on the road, and take whatever they are carrying. [Two Kings of Uganda, Robert Pickering Ashe (1889)]
He proposed an amendment giving the President carte blanche to muster all State banks, should he deem it necessary, into the Federal Reserve System. [New York Times (1933)]
Although designers will execute a job unilaterally if asked, most would rather not have carte blanche on either taste or price. [Kiplinger’s Personal Finance (1986)]
Normally reserved colleagues, typically thoughtful in-laws, usually polite tube commuters suddenly think they have carte blanche to play 20 questions. [Telegraph (2012)]