The candela (cd) is the base unit of luminous intensity in the International System of Units (SI). It is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

In simpler terms, the candela is the unit used to measure the brightness of a light source in a specific direction. It is based on the human eye's sensitivity to light at a specific wavelength (540 nm) and a specific angle (steradian).