1. In boxing/martial arts: any knee, elbow, forearm, foot, or hand/fist technique that connects from a relatively circular arc. The hidden "power" of the techniques is typically the hip rotation. Some styles emphasize more snap of the knee than others, but the hips generate the most stopping power from a physics equation alone. In this acceleration of rotary momentum, the technique is said to have it's greatest strength. Other joints may assist in the rotation from style to style, but generally don't due to telegraphing the technique. There are some techniques with a reverse principle in mind; even named as such (ei reverse-roundhouse kick, and otherwise known as hook kick).
2. In video games (commonly fighting game genre), the strongest of the character's kicks. (weak kick, strong kick, ROUNDHOUSE kick -- from a six-button mindset).
3. A house (residential building) without any corners.
1. The Muay Thai match was won by a roundhouse elbow to the opponent's temple.
2. The Sagat player beat the Ryu player with a roundhouse kick (third button kick).
3. You could say that Shelby lives in a roundhouse. Why? Because it's a cylinder-lighthouse.