Backlash is the small gap between meshing gear teeth when the gears are in contact. That clearance lets the gears carry lubrication films, tolerate manufacturing tolerances, and accommodate thermal expansion so the teeth don’t rub and bind. It’s an important parameter because too little backlash can cause interference and high wear, while too much backlash can lead to play, noise, and reduced precision. Measuring it is about quantifying that gap. The common method uses a dial indicator placed against a tooth and the gears are moved so one tooth can slide relative to the other; the reading on the dial (the amount of axial or radial movement at the pitch line) gives the backlash. Another way is to perform gear tooth play checks with a master gear or gauge to verify the gap lies within specified limits. The key idea is the measurement expresses how much free movement exists between meshing teeth when they’re in contact, not friction, timing, or oil color.