well until we have a definition of unit then it wire could fall under the definition of device. If you define device as something added to the wiring system then no wire is not a device.
2) an invention or contrivance, esp. a mechanical one, for some specific purpose
Screws and locknuts are "fittings" not devices, per Article 100.
Larry,Hmm..You are right, a "Locknut" (as in conduit or fitting type) maybe considered a fitting..bad choice for a word. I was thinking more like a nylock hardware fastener nut. How about just a nail?
A run of the mill screw (hardware fastener) is most certainly a device and not a fitting. A device does not necessarily have to have moving or multiple parts - it just has to have a purpose and a function.
From American Heritage:
A contrivance or an invention serving a particular purpose, especially a machine used to perform one or more relatively simple tasks.
Device. A unit of an electrical system that carries or controls electric energy as its principal function.
Fitting. An accessory such as a locknut, bushing, or other part of a wiring system that is intended primarily to perform a mechanical rather than an electrical function.