A dildo is a sexual device resembling an erect penis in shape, size, and overall appearance. Some expand this definition to include vibrators. Others exclude penis prosthetic aids, which are known as "extensions". Some include penis-shaped items clearly designed with vaginal penetration in mind even if they are not true approximations of a penis. Some people include devices designed for anal penetration (butt plugs) while others do not. These devices are often used by people of all genders and sexual orientations, for masturbation or for other sexual activity.