<operating system, compiler> To load and initialise the operating system on a computer. Normally abbreviated to "boot". From the curious expression "to pull oneself up by one's bootstraps", one of the legendary feats of Baron von Munchhausen. The bootstrap loader is the program that runs on the computer before any (normal) program can run. Derived terms include reboot, cold boot, warm boot, soft boot and hard boot.
The term also applies to the use of a compiler to compile itself. The usual process is to write an interpreter for a language, L, in some other existing language. The compiler is then written in L and the interpreter is used to run it. This produces an executable for compiling programs in L from the source of the compiler in L. This technique is often used to verify the correctness of a compiler. It was first used in the LISP community.
See also My Favourite Toy Language.