The candela is the SI base unit for luminous intensity. It is the brightness of a light source in a particular direction.
The candela is used to measure the brightness of light sources, like light bulbs or the bulbs in torches. It is the only SI unit based on human perception, as 'brightness' is something experienced in different ways by different people.
In the past, a 'standard' candle defined the candela. However, this standard varied from country to country, and from candle to candle, meaning it lacked sufficient accuracy. The candela, or 'the new candle', replaced these definitions in 1948 and it measures luminous intensity.
The definition of the candela will be updated to a new format of wording in 2019.