Definition
An aircraft engine starter that uses an electric motor to spin up a heavy flywheel to high speed, then mechanically engages (meshes) the spinning flywheel with the engine crankshaft so the stored rotational energy of the flywheel cranks the engine through its starting cycle.
Plain English
A starter that first uses an electric motor to spin a heavy wheel up to speed, then connects that spinning wheel to the engine to turn it over and start it.
Context Anchor
Seen in descriptions of older or large piston-engine aircraft starting systems, especially radial-engine aircraft.
Derivation
Inertia here means the tendency of a spinning mass to keep spinning. The flywheel stores energy as it is wound up by the electric motor; that stored energy is then released into the engine. 'Electric' distinguishes it from hand-cranked inertia starters, where the pilot or ground crew wound the flywheel up by hand.
Why Pilots Care
On aircraft equipped with this type of starter, the pilot must let the flywheel spin up fully before engaging it with the engine. Engaging too early gives a weak crank and a poor start; engaging properly delivers a strong, smooth turnover.
Analogy
Think of winding up a spinning top before letting it go. The electric motor does the winding; the flywheel does the work.
Intuition Check
Do not read this as just a normal electric starter. The key idea is stored spinning energy: the electric motor builds up the energy first, and the flywheel delivers it to the engine afterward.
Example Sentence 1
On the old radial trainer, the pilot held the start switch until the electric inertia starter wound up to a high-pitched whine before engaging it with the engine.
Example Sentence 2
On the vintage radial, the pilot waited for the inertia starter to reach full speed before releasing the clutch to turn the propeller.