Originally Posted by Ki

Definition of a sport: an activity involving physical exertion and skill in which an individual or team competes against another or others for entertainment.

Physical exertion in gaming: movement of arms and fingers and hands in quick succession. Being able to keep up with the pace of a quick team based game without losing concentration.

Skill: This one should be obvious.

Individual or team competes against another or others for entertainment: "MLG operates MLG.tv, the #1 online broadcast network for professional level competitive gaming; the MLG Pro Circuit, the longest-running eSports league in North America; and GameBattles, the largest cross platform online gaming tournament system with 9 million registered users across the globe."