A small, flat object used to mark the position of a golf ball when the ball is lifted on the putting green. The ball marker - which should be something about the size of a small coin - is place directly behind the golf ball before the ball is lifted. The ball is then replaced directly in front of the ball marker, on its original spot.
As noted, small coins are probably the most popular object to use as a ball marker, although many golfers also use items specifically manufactured for this purpose and that might carry a golf company logo or the name of a favorite course.
In the official rules, ball markers are addressed in Rule 20.
What's the right way to make use of your ball marker? See How to Mark the Golf Ball on the Putting Green.
For more about ball markers, see our FAQ:
Ball Markers: Do the Rules Specify What Should - or Shouldn't - Be Used?
Alternate Spellings: In the Official Rules of Golf, the term is hyphenated: ball-marker. Spelling it as one word - ballmarker - is also common.