Refers specifically to the individual keys on keyboard instruments like pianos and organs.
I tried pressing each key of the piano one by one.
This organ has many keys.
keystroke
white key
hidden mysteries; secret principle
lock and key; vital point
The kanji 鍵 originally means 'key' (as in a lock), and by extension came to refer to the keys of a musical instrument, likely due to their similar shape and function of 'unlocking' sound.