Mechanical or electronic devices, other than instruments, constructed to create or aid in the creation of musical sounds ... hardware sequencers (or devices which incorporate them) for example, the "stomp boxes" popular with guitarists, "loopers" used for Live looping, samplers ....

In other words, things which a musician might buy to enable or enhance a live performance, as opposed to gear designed for production.

Most stand-alone musical instruments probably belong in Category:Musical instruments. Some synthesizers, though, have features that fit this category.