A band worn over the sleeve, typically around the upper arm, to keep the sleeve in place. Were common in the US in the latter 19th century as ready-made shirts at the time only came with extra long sleeves. Most depictions of sleeve garters will typically be tied to outfits worn in the latter 19th century or early 20th century, such as outfits of barbershop quartets or US Prohibition Era bartenders.
If worn over the bare arm, then use Arm Garter instead.