A gladiator was a warrior during Roman times on Earth. Gladiators fought in ceremonial combat, often in an arena, and usually for the amusement or entertainment of an emperor and assembled crowd. Combat could involve multiple gladiators fighting each other or against wild animals.
The gladiatorial tradition was also present on the planet 892-IV in 2268 where, in an example of Hodgkin's Law of Parallel Planetary Development, the Roman Empire never fell but endured into a 20th century level of development. (TOS: "Bread and Circuses")