Rotor consisting of a rotating airfoil on the tail of a single-rotor helicopter; keeps the helicopter from spinning in the direction opposite to the rotation of the main rotor WordNet 3.0 & Open English WordNet

The tail rotor is a smaller rotor mounted vertically or near-vertically at the tail of a traditional single-rotor helicopter, where it rotates to generate a propeller-like horizontal thrust in the same direction as the main rotor's rotation. Wikipedia

Small tail-mounted helicopter rotor Wikidata

A small rotor mounted vertically at the end of the tail of a single-rotor helicopter so as to develop thrust in the same direction as the main rotor's rotation, to counter the torque effect created by the main rotor. Wiktionary

Small rotor. Wiktionary (translation)