On keyboard instruments, the rail which serves to stop the downward movement of the key. All keyboards have a keybed of some type. It’s essential that the rail be as close to perfectly straight as possible to ensure the travel of each key is consistent.
On traditional acoustic pianos the keybed is almost always made of a solid or multiple-ply hardwood. It includes a bumper, usually a thick felt material, to eliminate the “thump” when the key reaches the bottom of its travel.
Other instruments have employed different materials. The Rhodes electric piano’s keybed was an aluminum bar with a plastic bumper (which, interestingly, was manufactured by Wurlitzer for several years). Even synth action keyboards, which replace traditional weights with a spring return mechanism, have a “stop” bar (often made of plastic).
In recent years the term “keybed” has been used to describe the entire piano action – the keys, weights, etc. – as opposed to just the rail.