Definition
An engine-driven air compressor mounted between the carburetor or fuel injection system and the intake manifold of a reciprocating engine. It compresses the fuel-air mixture (or air, in injected systems) before it enters the cylinders, increasing manifold pressure and allowing the engine to maintain power at higher altitudes where ambient air is thinner.
Plain English
A pump inside the engine system, driven by the engine itself, that squeezes the air going into the cylinders so the engine can still make good power when the air outside gets thin at altitude.
Context Anchor
Seen in aircraft engine systems, especially older or high-performance piston engines that use a built-in blower to help maintain power at altitude.
Derivation
Internal' here means located within the induction system downstream of the carburetor, as opposed to an 'external' supercharger or turbocharger fitted upstream. 'Supercharger' comes from 'super-' (above, beyond) and 'charge' (the fuel-air mixture loaded into the cylinder), so it literally means a device that loads more charge into the cylinder than the engine could draw in on its own.
Why Pilots Care
It delivers higher manifold pressure and usable power at altitude without the lag associated with turbochargers, improving climb and high-altitude cruise performance.
Intuition Check
“Internal” does not mean the compression happens inside the cylinders. Here it means the supercharger is built into the engine rather than being a separate external unit.
Example Sentence 1
The radial engine's internal supercharger let the aircraft maintain cruise power well above the altitude where a normally aspirated engine would have started losing performance.
Example Sentence 2
During the engine run-up the internal supercharger produced the expected rise in manifold pressure when the throttle was advanced.