Plain English Meaning
A system that uses a turbocharger to keep the engine producing the same power at altitude that it would produce at sea level. It does not boost the engine beyond normal power; it just stops power from dropping off as the airplane climbs.
Full Definition
A turbocharging system designed to maintain sea-level manifold pressure as the airplane climbs, rather than to increase manifold pressure above the engine's normal sea-level value. As altitude increases and outside air thins, the turbocharger compresses more of it to keep manifold pressure at, or near, the same value the engine would produce on the ground. Also called normalizing.