A disk of high-quality quartz or glass with at least one surface polished to be extremely flat, typically within a few nanometers. When placed against a machined surface under monochromatic light, it creates interference fringes. Metrologists count these fringe bands to calculate the exact deviation from perfect flatness.
Heard on the job
Put the optical flat on the micrometer anvil and count the fringes to check for wear.