A mulligan, most simply put, is a "do-over." Hit a bad shot? Take a mulligan and replay that stroke.
So as you've probably guessed, a mulligan is never "legal" under the Rules of Golf. Mulligans are most often employed during friendly rounds by golf buddies; or during charity or playday tournaments where mulligans are sometimes sold. If mulligans are for sale, that means the golfer can buy, say, three mulligans for a set price each. The sale of mulligans is sometimes used as an additional fund-raiser at charitable events.
Are there rules governing the use of mulligans? No - whatever a group of golfers agrees upon is what counts (unless you are using mulligans in something like a charity tournament or association outing setting - then do what the organizers tell you).
The most common mulligan customs, however, include using mulligans only on the first tee; or limiting the use of mulligans to one per nine holes. It's most common for mulligans to be used only off the tee; some groups might also allow mulligans from the fairway. It is rare for mulligans to be seen on the putting green.
As with all recreational golf customs, the use of mulligans varies from place to place. They are common in the United States, but rarely seen in the UK, for example. As with all localized golf customs, formats and betting games, clear up the ground rules before play starts to avoid possible confusion later.
See also: Why are mulligans called that?
Also Known As: Mullie, Sunday ball, lunch ball
Examples: Bob's drive was terrible, so he took a mulligan.
"Are we playing mulligans today?" "Yes, one mulligan per nine, but only off the tee."