Mechanical keyboard switch
Also called key switch
A mechanical keyboard switch is an individual spring and contact assembly under each key. Each switch can fail on its own, which is why one dead key on a mechanical keyboard is often repairable.
In practice
Because each key has its own switch, a mechanical keyboard tends to fail one key at a time rather than all at once. The usual failures are a key that stops registering, one that doubles and one that feels gritty. Switches come in different types with different feel, but they fail in similar ways and a single bad switch doesn’t mean the keyboard is finished.
How to check
Press every key in the keyboard test and note any that stay unlit or count twice. Pull the keycap of a suspect key and look for debris. On a hot swap keyboard the switch pulls out with a tool and a spare goes in. On a soldered board, replacing a switch needs desoldering, which is a bigger but still common repair.