A user of a computer system with unrestricted access to all functionality. The canonical example of this is the root user on unix systems.

superuser n.

[Unix] Syn. root, avatar. This usage has spread to non-Unix environments; the superuser is any account with all wheel bits on. A more specific term than wheel.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.