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.
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).
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".
4. [MIT] n. A student who studies too much and hacks too little. (MIT's student humor magazine rejoices in the name 'Tool and Die'.)