A hole in one is also known as an ace. The plural is "holes in one," and the term is often spelled with hyphens: hole-in-one.
One of the earliest stars of professional golf, Young Tom Morris, scored the first-recorded hole-in-one in 1868.
Your chances of making a hole in one improve the better you are as a golfer, and the shorter the hole you are playing. After all, the first step in making an ace is reaching the green with your tee shot. So holes-in-one are far, far more likely to occur on par-3 holes. (Statistically, virtually all holes-in-one happen on par-3 holes; par-4 aces and especially par-5 aces are exceedingly rare, although they do happen - see below.)
Holes-in-one are scarce; they are not a common site in golf, but they do happen for golfers of all skill levels. Luck plays a very big part in scoring an ace, too, and there are many stories of golfers who make holes-in-one right after learning how to swing a club.
As an example of how the unlikeliest of golfers sometimes get incredibly lucky, there's the story of the golfer who had never birdied before yet made two aces in the same round (with instructor Rick Smith looking on).
See hole-in-one odds for more about the odds of making an ace.
Also see:What's the best shot you've ever hit?