A reboot is the process of shutting down all running software on a machine, including the operating system itself, and then immediately reloading as if the machine had just been powered on.
A reboot differs from actually powering down the machine and turning it back on only in that the power is never turned off. The software simply acts as if the computer was shutting down completely, and then immediately acts as if it had just been powered on.
Reboots are common debugging and system-stabilization techniques, as they force the entire system back to a known initial state. (A power-off shutdown also includes resetting the hardware to a known initial state as well.) More information: Why do so many tech support solutions start with “reboot” or “turn it off”?
In computing, rebooting is the process by which a running computer system is restarted, either intentionally or unintentionally. Reboots can be either cold (alternatively known as hard), in which the power to the system is physically turned off and back on again causing an initial boot of the machine, or warm (or soft) in which the system restarts without the need to interrupt the power. The term restart is used to refer to a reboot when the operating system closes all programs and finalizes all pending input and output operations before initiating a soft reboot.