A "reboot loop" refers to a situation where a device or computer continuously restarts itself without being able to fully turn on or stay on. It's like getting stuck in a cycle of restarting over and over again. Full definition
Some people are getting stuck in an endless reboot loop, some can't even use the power button to boot out of «rest mode» and into «safe mode». (bytesizeimpressions.com)
The initial rollout of the Anniversary Update caused some PCs to freeze after completing the upgrade, resulted in reboot loops for others, and broke an important enterprise feature. (zdnet.com)
But in my case, the laptop just went into an endless reboot loop. (thewindowsclub.com)