Phasor

A phasor, in electrical at least, is just a way of representing at a single point, the voltage or current of an AC circuit.

Example
Phasors are just vectors but for the purposes of power diagrams they are some unique properties:
 * All phasors rotate anticlockwise.
 * We pretend that phasors aren't moving. For this reason the absolute position doesn't matter just the relative.

All of this means that the

Summary

 * All phasors rotate anticlockwise
 * Phasors are denoted in bold. A for example represents a phasor.

Phasor Diagram
A phasor diagram is a snapshot of rotating phasor taken at any time instant.

The time-invariant is technically due to complex number things, but really it is because in a power diagram, all phasors are rotating at the same frequency (50Hz), which means that they are effectively stationary between each other.