The moment at which the spark plug fires, usually expressed in the number of crankshaft degrees before the piston reaches the top of its stroke.
It is important that the spark coming from the spark plugs ignites the air-fuel mixture at the correct moment determined by the manufacturer of the engine. Often this is a few degrees before top dead center (BTDC). Suppose the specs indicate that it should be set at 5 degrees BTDC, but in actuality it is set at 6 degrees. In that case, the timing is advanced. If it were set at 4 degrees, the timing is retarded. Incorrect ignition timing may result in poor performance and excessive fuel consumption.
Assembling the ignition components (either Magneto or Distributor) ignition such that the spark is delivered to each cylinder at nearly the top of its compression stroke. The timing will usually be at slightly beyond TDC. The terms Advancing and Retarding the timing refers to the relationship of when the spark plug fires to TDC. Ignition timing is different at different RPMs. There is usually a device to advance the timing as the RPMs increase. The faster the engine spins the sooner it needs to fire.