XO stands for Extra Old. XO Cognacs are made only from eaux-de-vie at least ten years old.
Historically the minimum age was six years old, but it was increased to 10 years in April 2018. While it sounds like a dramatic hike, for many producers it was not much of an issue as XOs often have a much older average age (numerous XO Cognacs from the big houses are 10-15 years old or more). Cognacs such as "Napoleon" or "Old Reserve" are terms that are considered equivalent to XO Cognac.