Timing mark on pulley at 6° before TDC.
A timing mark is a mark used for setting the timing of the ignition system of an engine, typically found on the crankshaft pulley (as pictured) or the flywheel, being the largest radius rotating at crankshaft speed and therefore the place where marks at one degree intervals will be furthest apart.
On older engines it is common to set the timing using a timing light, which flashes in time with the ignition system (and hence engine rotation), so when shone on the timing marks makes them appear stationary due to the stroboscopic effect. The ignition timing can then be adjusted to fire at the correct point in the engine's rotation, typically a few degrees before top dead centre and advancing with increasing engine speed. Modern engines usually use a crank sensor directly connected to the engine management system.
The term can also be used to describe the tickmarks on the side of an optical mark recognition sheet, used to confirm the location of the sheet as it passes through the reader.