Device to vary the point at which the spark plug fires in relation to the piston's position in the cylinder.

the timing of ignition relative to the position of the piston in an internal-combustion engine

The number of degrees of crankshaft rotation before TDC where the spark plug is fired (see ignition timing.)

To adjust the ignition timing so that the spark plugs will fire sooner than they have been firing.

The moving ahead of the ignition spark in relation to the piston position.