Plain English Meaning
When something is moving in more than one way at once, the resultant velocity is the overall speed and direction you get when you add those motions together.
Full Definition
The single combined velocity that results when two or more separate velocities act on the same object at the same time. In the context of P-factor, it is the combined velocity seen by a propeller blade, made up of the blade's rotational speed plus the aircraft's forward airspeed.
Context Anchor
Seen in discussions of P-factor and asymmetric propeller loading, especially when explaining why one side of the propeller can produce more thrust than the other.
Derivation
From Latin 'resultare' meaning 'to spring back' or 'to follow as a consequence.' In physics it came to mean the single value that 'results' from combining several. That fits the use: one combined velocity that follows from adding the separate ones together.