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)