Definition
A physical button on a Garmin GPS navigator (such as the GNS 430/530 series) labeled MENU, which when pressed opens a list of options or actions available for the page currently displayed. It is called a hard key because it is a real, fixed button on the unit's bezel, as opposed to a soft key whose function changes depending on what is shown on the screen.
Plain English
A real button on the GPS labeled MENU. Press it, and the GPS shows you a list of things you can do from whatever page you are currently looking at.
Context Anchor
Seen when using GPS pages, especially when the handbook tells you to press MENU while viewing the Nearest Airports page.
Derivation
Hard key means the button is physically built into the unit and always does the same thing, in contrast to soft keys, which are buttons whose labels appear on the screen and change with the page. The word menu is borrowed from restaurant use — a list of choices available right now.
Why Pilots Care
Knowing which buttons are hard keys versus soft keys speeds up cockpit work. The MENU hard key is a consistent way to access page-specific options without hunting through the screen, which matters when workload is high or attention needs to stay outside the cockpit.
Analogy
It is like the MENU button on a TV remote: the button stays in the same place, but the choices that appear depend on what screen you are using.
Intuition Check
Hard does not mean difficult here. It means the MENU control is a physical button with a fixed role.
Example Sentence 1
On the Nearest Airports page, press the MENU hard key to bring up options such as setting runway length and surface filters.
Example Sentence 2
Pressing the MENU hard key on the GPS allowed selection of additional setup options during the flight.