When the geometry of 3D models intersect with each other. Generally the result of poor model placement and lack of morphing effects but sometimes, it's intentional and necessary.

Back in the late 1990's, this was used extensively in video games to compensate for low polygon counts. It's still used today for characters like Kirby, whose "arms" and feet are separate models partially clipping into the main body due to the difficulty in animating him properly as one single model.