1. Someone who works well in a team or group. John's always been a real team player, never hesitating to chip in or help others out where he can.

2. Someone who does what is necessary to maintain the success or further the goals of their team or company, especially at their own expense or to their own detriment. The boss will always try to belittle you for not being a team player if don't agree to stay late or work weekends.