Lens Mount
The mechanical and electronic interface that couples a lens to a camera body.
In depth
A mount is defined by three things: its diameter, its flange focal distance, and its electronic protocol. Diameter constrains how fast a lens can be at the corners; flange distance determines what can be adapted onto it; the protocol governs autofocus speed, aperture control and stabilisation communication. Changing a mount obsoletes lens investment, which is why manufacturers do it perhaps twice a century.
Examples in practice
A 54 mm mount diameter with a 20 mm flange distance allows lens designs, like an f/0.95 standard prime, that a narrower mount cannot support.