Boot is the early process of starting the machine and loading the operating system.
People often use boot and startup as if they are exactly the same. Boot is part of the larger startup story.
Boot is the startup stage where the machine begins loading the operating system after the first hardware checks.
Boot is the early wake-up path that starts the computer system.
Boot is the early part of system startup where the machine begins loading the operating system.
If the screen lights up, people may think boot succeeded fully. But a machine can power on and still fail during or after boot.
Study the larger concept after the word itself becomes clear.