Definition
A system that routes engine-heated air into the carburetor intake to prevent or remove ice that can form inside the carburetor when moist air cools as it passes through the venturi and across the throttle plate. The pilot activates it with a control in the cockpit (commonly labeled 'carburetor heat' or 'carb heat'), which redirects induction air through a shroud around the exhaust before it enters the carburetor.
Plain English
A control that sends warm air into the carburetor to melt or prevent ice that can build up inside it and choke the engine.
Context Anchor
You will encounter this term in engine controls and engine operation discussions for airplanes equipped with carburetors, often when learning when to use carburetor heat.
Derivation
Pre-' means before, and 'heater' means a device that warms. So a 'preheater' warms the air before it reaches the carburetor — heating it ahead of time so ice cannot form once it gets there.
Why Pilots Care
Carburetor ice can cause partial or total loss of engine power; the preheater provides the only in-flight means to restore full power without shutting down the engine.
Intuition Check
Do not think of the carburetor preheater as a cabin heater or as something used only before engine start. In this context, it heats intake air for the engine during operation to prevent or clear ice in the carburetor.
Example Sentence 1
Before reducing power for the descent, the pilot pulled the carburetor preheater to prevent ice from forming during the low-power glide.
Example Sentence 2
On the ground run-up, the pilot verified that selecting carburetor preheater produced the expected RPM drop.