A short program loaded from non-volatile storage and used to bootstrap a computer.
On early computers great efforts were expended on making the bootstrap loader short, in order to make it easy to toggle in via the front panel switches. It was just clever enough to read in a slightly more complex program (usually from punched cards or paper tape), to which it handed control. This program in turn read the application or operating system from a magnetic tape drive or disk drive. Thus, in successive steps, the computer "pulled itself up by its bootstraps" to a useful operating state.
Nowadays the bootstrap loader is usually found in ROM or EPROM, and reads the first stage in from a fixed location on the disk, called the "boot block". When this program gains control, it is powerful enough to load the actual OS and hand control over to it. A diskless workstation can use bootp to load its OS from the network.