(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 arenaof debate; the arena of life.

(3):

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