Wiktionary
n. (context computing Microsoft Windows English) A troubleshooting mode of (w: Microsoft Windows) in which most advanced functions are not activated.
Wikipedia
Safe mode is a diagnostic mode of a computer operating system (OS). It can also refer to a mode of operation by application software. Safe mode is intended to help fix most, if not all problems within an operating system. It is also widely used for removing rogue security software.
Safe mode is an operating mode of a modern spacecraft during which all non-essential systems are shut down and only essential functions such as thermal management, radio reception and attitude control are active.
Usage examples of "safe mode".
There were two separate braking systems on the cars, each with its own fail-safe mode.
Within a few days, all of the people in our little company were retrained, barring two of the janitors (a musicologist and a history major, who weren't interested in technical things), and within a few weeks, all of our old machinery and weapons were operational in the new, safe mode.
We've got to cut the ground That will short out the circuit into its fail-safe mode without tripping the field.