Redline refers to the maximum speed at which an internal combustion engine and its components are designed to operate without causing damage to the components themselves or other parts of the engine. The redline of an engine depends on various factors such as stroke, displacement, composition of components, and balance of components. Redlines vary anywhere from a few hundred RPM (very large engines, i.e. trains, generators) to in the tens of thousands of RPM (smaller, usually high performance engines such as motorcycles and sports cars). Diesel engines normally have lower redlines than comparatively-sized gasoline engines.

Redline is usually determined by stress-testing the engine and its components. Changing the stroke of the engine (the lateral distance the pistons travel) can also change the redline. Engines with short strokes can handle higher RPM because there is less force in reciprocating motion (see reciprocating engine). Lighter components can increase the redline as well, since they have less inertia and decrease forces present in the engine.

The actual term redline comes from the red bars that are displayed on tachometers in cars starting at the RPM that denotes the redline for the specific engine. Operating an engine in this area is known as redlining. Straying into this area usually does not mean instant engine failure, but may increase the chances of damaging the engine. Most modern cars have computer systems that prevent the engine from straying too far into the redline by cutting fuel flow to the fuel injectors/carburetor or by disabling the ignition system until the engine drops to a safer operating speed.