What is a Sport?

Here is an exceedingly narrow definition sure to provoke some arguments:

A sport is a contest of physical prowess characterized by scoring with symmetrical objectives, offense and defense, with the purpose of entertainment.  This excludes all marksmanship events, races, and indirect competitions.  Track and field, golf, basically anything you can have a world record for doesn't count because if defense was meaningfully part of the competition, we wouldn't be able to compare achievements.