Americans, Canadians, New Zealanders, Brits, and the Irish all call it Diet Coke. Australians use the slightly longer name of Diet Coca-Cola.
However, the refreshing drink is known as Coca-Cola Light across mainland Europe, Malaysia, and Mexico, while in Sweden it's referred to as Light Coca-Cola.
The logo and branding are the same worldwide, however, so it's easy to spot the distinctive silver cans wherever you are.