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.
Plain English
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.
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.