Booting
Booting is what happens when a computer starts. This happens when the power is turned on. It is called "reboot" if it happens at other times. When you boot a computer, your processor looks for instructions in system ROM (the BIOS) and executes them. They normally 'wake up' peripheral equipment and search for the boot device. The boot device either loads the operating system or gets it from somewere else.
People use the word "boot" to mean "to start a computer" or other device with electronics built in. For example, if a person wants to ask a friend to turn on a satellite phone, they would say "could you boot up the satellite phone?".
Most operating systems call the first data storage device it uses a boot device. It may also be called a bootstrap loader. This is because the computer is making itself go, as in the idiom. When we start a computer, we can often see the simple instructions the computer uses to start, then more complicated pictures or software.
The phrase "to boot" in this meaning is short for "to bootstrap". This use is part of net jargon along with similar multi-use words like Internet or web. Often the computer is just called a box, so a phrase like "to boot the box" means "to start the computer".
The term "reboot" can also be used in a different context to mean a restarting of a storyline established in previous iterations of a series of fiction.
Booting Media
Switches and cables used to program ENIAC (1946)
Initial program load punched card for the IBM 1130 (1965)
IBM System/3 console from the 1970s. Program load selector switch is lower left; Program load switch is lower right.
PDP-8/E front panel showing the switches used to load the bootstrap program
An Intel 2708 EPROM "chip" on a circuit board
Windows To Go USB Drive
An unlocked bootloader of an Android device, showing additional available options
