1. n.A program used primarily to create, manipulate, modify, or analyze other programs, such as a compiler or an editor or a cross-referencing program. Oppose app, operating system; see also toolchain.
2. [Unix] An application program with a simple, ‘transparent’ (typically text-stream) interface designed specifically to be used in programmed combination with other tools (see filter, plumbing).
3. [MIT: general to students there] vi. To work; to study (connotes tedium). The TMRC Dictionary defined this as “to set one's brain to the grindstone”. See hack.
4. n. [MIT] A student who studies too much and hacks too little. (MIT's student humor magazine rejoices in the name Tool and Die.)