Boot

"To boot up" a computer is to load an operating system into the computer's main memory or random access memory (RAM). Once the operating system is loaded (and, for example, on a PC, you see the initial Windows or Mac desktop screen), it's ready for users to run applications. Sometimes you'll see an instruction to "reboot" the operating system. This simply means to reload the operating system (the most familiar way to do this on PCs is pressing the Ctrl, Alt, and Delete keys at the same time).