Definition
A combined electrical machine, common on turbine-powered aircraft, that functions first as a starter motor to spin the engine up to self-sustaining speed, then automatically switches role to act as an engine-driven generator that supplies DC electrical power to the aircraft systems and recharges the battery.
Plain English
One single unit that does two jobs. First it acts like an electric motor to get the engine spinning so it can start. Once the engine is running, the same unit is driven by the engine and produces electricity for the aircraft.
Context Anchor
Seen in aircraft electrical system and engine-starting discussions, especially on turbine-powered aircraft.
Derivation
The name simply joins the two functions the unit performs: starter (gets the engine turning) and generator (produces electrical power). It exists because combining both jobs into one device saves weight, space, and complexity compared with fitting two separate machines.
Why Pilots Care
Combines two functions in one lightweight unit, reducing complexity and weight on many light aircraft and helicopters.
Intuition Check
Do not think of a starter-generator as two separate parts simply mounted together. In this use, it is one electrical machine that changes role depending on whether the engine is being started or is already running.
Example Sentence 1
After the turbine reached self-sustaining speed, the starter-generator dropped offline as a motor and came back on as the main DC generator.
Example Sentence 2
After engine start the starter-generator supplied electrical power to the bus.