A roundhouse kick is a kick

Kick

In combat sports and hand-to-hand combat, a kick is a physical strike using the foot, leg, or knee . This type of attack is used frequently, especially in stand-up fighting...

 in which the attacker swings his leg around in a semicircular motion, striking with the front of the leg or foot. This type of kick is utilized in many different martial arts

Martial arts

Martial arts are extensive systems of codified practices and traditions of combat, practiced for a variety of reasons, including self-defense, competition, physical health and fitness, as well as mental and spiritual development....

 and is popular in both non-contact and full-contact martial arts competitions. The kick has many variations based on stance, leg movement, striking surface, and the height of the kick.

A semi-circular kick is a round kick to forty five degree roundhouse kick (or "diagonal kick").