Definition: A fluid ounce (fl oz) is a unit of volume used for liquids. There are two different definitions: the US fluid ounce and the Imperial fluid ounce, which differ slightly in volume.

History/origin: The fluid ounce has roots in ancient Roman measuring systems. It was later adopted into the Imperial and US customary systems, although the definitions diverged.

Current use: The fluid ounce is widely used for measuring liquids in both the United States (US fl oz) and the United Kingdom (Imperial fl oz), although the metric system is becoming increasingly common.