Ignition timing refers to the precise moment when the spark plug in an internal combustion engine fires to ignite the air-fuel mixture. This timing is crucial because it affects engine performance, efficiency, and emissions. If the spark occurs too early or too late, it can lead to issues like knocking, reduced power, or increased fuel consumption. Proper ignition timing is typically measured in degrees before or after top dead center (TDC), which is the point where the piston is at its highest position in the cylinder. Adjusting ignition timing can optimize engine performance for different conditions, such as varying speeds or loads.