A strap on is an artificial phallic appendage or dildo that can be strapped on to allow someone to have penetrative sex. Strap ons can also be used by people with penises to provide a harder/better erection or to provide the person's partner with a different penetrative experience than their biological penis provides. These are often referred to as male strap ons.
A strap on is attached with a belt or harness and are usually designed to sit in a similar position as a biological penis. However, some strap-on harnesses eschew the straps for a sturdy, comfortable pair of underwear with a dildo attachment in the front that allows for more comfortable long-term wear.
Strap-ons may be used for penetrative vaginal sex, anal sex, or pegging. Some people also enjoy giving a strap-on blow job. Although the recipient can't feel the fellatio, the act does provide a powerful visual.
When buying a strap-in, look for high-quality materials, fit and comfort and the how the dildo is attached.
Strap ons may also be referred to as strap-on dildos.