A metric that estimates the percentage of his team's possessions a player "uses" while he is in the game. As players "use" more possessions, their overall efficiency tends to drop.
In an effort to determine how much impact players have on their teams, sports statisticians have developed metrics such as Usage Percentage. Examining Usage Percentage gives us an indication of how efficient a player is given the amount of possessions he uses.
What defines a quality player is someone who can have a high Usage Percentage, but still plays at a high rate of efficiency. Teams can look at the Usage Percentage of players on their team, and determine how to balance usage across their lineup to maximize team efficiency.
Although the formula itself looks a bit more complicated, the basic idea is to look at a player's combination of field goal attempts, free throw attempts and turnovers, and find the percentage of the team totals he uses in those same categories.