Jean-Leon Gerome's 1872 painting Pollice Verso. Murmillo looks toward the ediotr (emperor) to decide opponent's fate.
In Roman times, at the games, an editor was an overseer or "referee" of the gladiator fights. Their job included giving talks at the opening and closing of the games, announcing who was fighting and to choose if a defeated gladiator was to live or die.
Usually the role was held by a magistrate, one of the government officers of the Roman republic. At times lanistas, such as Batiatus or Vettius would play the role, though usually due to the lack of a magistrate being present.
Numerius, condemning Varro to his death.
Editors also existed for smaller fights, although the term editor would be used very lightly. Numerius, for his 15th birthday, has Spartacus and Varro demonstrate a fight in Batiatus' villa. Although originally intended to be an exhibition match, when asked to pass judgement on the fallen warrior, he extends his arm and shows his thumb down, meaning that Varro was to die.