Single phase Ac Wire voltages
The neutral wire is connected to an Earth Ground somewhere, and is therefore defined to be "Zero Volts".  The voltage on the phase (or "live"  or "line") wire varies/alternates between positive and negative relative to the Neutral wire.

AC normally implies a regular alternating voltage (50 or 60Hz for Mains Power, but any frequency for other uses).

A pulse signal is normally at one voltage (often, but not necessarily, zero), but occasionally changes to another voltage for a short time.