Noah Webster's New International Dictionary of the English Language

1. (n.) The area in the central part of an amphitheater, in which the gladiators fought and other shows were exhibited; -- so called because it was covered with sand.

2. (n.) Any place of public contest or exertion; any sphere of action; as, the arena of debate; the arena of life.

3. (n.) Sand or gravel in the kidneys.