1. The supervisor program or OS on a computer.
2. The entire computer system, including input/output devices, the
supervisor program or OS, and possibly other software. 3. Any
large-scale program. 4. Any method or algorithm. 5. `System
hacker': one who hacks the system (in senses 1 and 2 only; for
sense 3 one mentions the particular program: e.g., `LISP hacker')