Here's how the neutral was defined by the British IEE some years ago:
Neutral conductor. The neutral conductor of a 3-phase 4-wire system, the conductor of a single-phase or d.c. installation which is earthed by the supply undertaking (or otherwise at the source of supply), or the middle wire or common return conductor of a 3-wire d.c. or 3-wire single-phase a.c. system.
Note the circular reference in the first part of the definition!
[This message has been edited by pauluk (edited 12-14-2003).]