A boot device is used to start a computer. It is named after a boot which fits on the foot. The word bootstrap is also closely related, and means, to use something simpler to get something more complex to make itself work better. It comes from the English phrase "pull yourself up by your own bootstraps."
Before a computer can operate normally, it must have operating system instructions that tell it how to perform basic functions. A boot device loads the operating system into the memory of the computer.
Devices that can boot a computer are usually boot disks or boot drives (normally a hard drive or Solid State Drive, but can be a floppy disk, flash drive or a CD). Some network computers use boot chips that get the operating system over a network. Web phones also use such chips to identify the user to the mobile phone network. Boot card standards may let many users boot kiosk computers with full privacy and access to all application software they own. There are also boot boards or boot add-in cards that are more permanent than boot cards.
The boot in boot device is the same as booting (or starting up). This is short for bootstrapping, or to start with simple stuff and make complex stuff out of it.