Keybed

[T]he pressure required on the keybed to hold a note down is not necessarily more than will suffice to sound the note at its softest . . .

Source: wiktionary

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.

Source: wiktionary

The keybed is heavily constructed to remain flat when exposed to humidity and temperature changes. . . . Upon the keybed is positioned the assembly which holds the keys and action in place; this is called the key frame.

Source: wiktionary

In recent years the term “keybed” has been used to describe the entire [electronic/digital] piano action – the keys, weights, etc. – as opposed to just the rail.

Source: wiktionary