game

A playful or competitive activity.

A video game.

(nearly always singular) A field of gainful activity, as an industry or profession.

"He's in the securities game somehow."

To gamble.

To play card games, board games, or video games.

To exploit loopholes in a system or bureaucracy in a way which defeats or nullifies the spirit of the rules in effect, usually to obtain a result which otherwise would be unobtainable.

"We'll bury them in paperwork, and game the system."

Willing to participate.

(of an animal) That shows a tendency to continue to fight against another animal, despite being wounded, often severely.

Persistent, especially in senses similar to the above.