Plain English Meaning
A button whose job changes depending on what the screen tells it to do. The screen shows a small label next to each button, and that label tells you what pressing it will do right now.
Full Definition
A button on the bezel of a glass cockpit display whose function is not fixed but is defined by a label shown on the screen directly above or beside it. The label, and therefore the button's function, changes depending on the page or menu currently displayed.
Context Anchor
Seen when using glass-cockpit menus, such as selecting the Nearest Airports page on the primary flight display.
Derivation
Called 'soft' because the button's function is set by software, not hardwired. A 'hard key' always does the same thing; a soft key does whatever the screen says it does at that moment.
Why Pilots Care
Allows a small number of physical buttons to control many different functions without adding clutter to the instrument panel.
Intuition Check
A soft key is not a physically soft button, and it does not mean you press it gently. It means the button’s function is controlled by the screen and can change.
Example Sentence 1
On the PFD, press the NRST soft key to bring up the list of nearest airports.
Example Sentence 2
After selecting the Direct-To page, new soft-key options appeared along the bottom of the display.