The best number that's been posted by a professional golfer during an 18-hole round in tournament play. Most of these courses were par 72, so that is usually 13 under par. If a golfer were to birdie each hole, he would shoot 54 on a par 72 course.
This is next to impossible because a birdie is the equivalent of hitting a home run in professional baseball or serving an ace in professional tennis. A human just can't keep the concentration to perform at the "excellent" level for the four hour period of time it takes to play a round of golf, play a ball game, or play a match in tennis.
(I just noticed that four hours is about the same in each, isn't it? Conspiracy theory?)