In Latin de facto means according to fact, and this is roughly what it means in English. It’s defined as in reality or fact, but its de facto definition is closer to serving a function or filling a role without being officially assigned to that function or role. For example, a de facto leader of a group is someone who has not been officially designated leader yet functions in that role. De facto is sometimes contrasted with de jure, which means according to law or officially.
De facto has been in English for several centuries. So, like most established loanwords, it does not need to be italicized in normal use (we italicize because it’s presented as a word out of context).
Yet the American Bible incorporates a de facto Third Testament, which validates this assertion of American uniqueness. [Los Angeles Times]
Last weekend, residents of Chamonix, the French resort that is the de facto capital of European skiing, awoke to find up to 20cm of fresh snow. [Financial Times]
Government flags are government property, and Williams was the premier, so perhaps he had de facto authority to do what he wanted. [Winnipeg Free Press]
He won’t stop campaigning because he wants to make the next election into a de facto referendum on the carbon tax. [The Canberra Times]