Commonly used in compound words like ショルダーバッグ (shoulder bag) or ショルダーパッド (shoulder pad). As a standalone noun, it refers to the shoulder area, often in fashion or accessory contexts.
This shoulder bag is light and easy to use.
I'm wearing a jacket with shoulder pads.
shoulder pad
shoulder phone
shoulder bacon
drop shoulder
keytar
Borrowed from English 'shoulder'.