ludus

Ludus, ludi, m., second declension, and the number changes the meaning. In the plural, ludi are the public games: days of chariot racing, theatre and beast shows, paid for by a magistrate who wanted votes.

In the singular it is play, a joke, or - the sense that surprises people - a school, since ludus litterarius is where Roman children were drilled in their letters. A gladiator training school is a ludus too, which makes the semantic range fairly wide for one small word. The verb is ludo, ludere, lusi, lusum, and the family gives English illusion, allude, prelude, collude and ludicrous.