Plain English Meaning
The amount of work per second that actually goes into moving the airplane forward, rather than being lost as heat, noise, or wasted air motion.
Full Definition
The useful power an aircraft's powerplant delivers to move the airplane forward through the air, equal to thrust multiplied by velocity. It represents the rate at which the engine and propeller (or jet) actually do work pushing the aircraft through the air, as distinct from the total power produced by the engine itself.
Context Anchor
Seen in range performance discussions, especially when comparing how much useful forward motion the airplane gets from the fuel it burns.
Derivation
From Latin propellere, 'to drive forward.' Propulsive refers to whatever drives the aircraft ahead, so propulsive power is the share of the engine's power that genuinely pushes the airplane through the air.