To load an operating system into memory, usually from a hard disk, although occasionally from a floppy disk. Booting is generally an automatic procedure that begins when you turn on or reset your computer. A set of instructions contained in ROM begins executing. The instructions run a series of power-on self tests (POSTs) to check that devices such as hard disks are in working order, locate and load the operating system, and finally pass control over to that operating system.
Boot may be derived from the expression "pulling yourself up by your own bootstraps" and is sometimes called bootstrap.