Part of a wheel hub that holds the wheel to the dropouts.
This tag is for all types of Axles.
You could also use [thru-axle] if your wheel has a shaft that screws in like an oversized bolt. Also known as Through Axle.
Many axles are [quick-release] based and have a [skewer] through a hollow tubular axle to retain them.
Older bikes and some BSOs have simple wheel nuts that thread onto a solid bolt-like shaft.