Hot Shoe
The standardised bracket on top of a camera that both holds an accessory and provides electrical contacts to trigger it.
In depth
The central pin is the universal sync contact, unchanged for decades, so almost any flash will fire on almost any camera. The surrounding contacts are proprietary and carry the through-the-lens metering, high-speed sync and remote control data. Modern shoes increasingly carry digital audio and power for microphones and accessories too.
Examples in practice
A third-party flash fires reliably on any camera via the centre pin, but only a matched unit gets automatic TTL exposure.