Causing the computer to start executing instructions. PCs and Macs contain built-in instructions in a ROM or flash memory chip that are automatically executed on startup. These instructions search for the operating system, load it and pass control to it. Starting up a large computer may require more button pushing and keyboard input.

Put Your Boots On!
The term comes from "bootstrap." Since bootstraps help you get your boots on, booting the computer helps it get its first instructions. The term is often used erroneously for application software. For example, you might hear someone say "let's boot Excel," whereas the correct usage is "launch Excel" or "load Excel." See cold boot, warm boot, clean boot and first boot sequence.