<p><strong>Noun</strong></p><ul><li>Optics: a basic unit of luminous intensity in the International System of Units (SI), formally defined as the luminous intensity of a source that emits monochromatic radiation of frequency 540 × 10^12 hertz and that has a radiant intensity of 1/683 watt/steradian; adopted in 1979 as the international standard of luminous intensity.</li><li>Symbol: Cd</li></ul>