An element discovered by the English chemists Sir William Ramsay and Morris Travers in 1898, using fractional distillation. It is one of the noble gasses. Its symbol is Kr, and its atomic number is 36.

He took the name from the Greek word kryptos, meaning hidden.

Its main use in is neon lights, in which it is used to make a pale violet. It is also often used for airport runway lights. Since it also has a fast response time to electric currents, it is often used with xenon in high-intensity, sort-exposure photographic flash bulbs and in strobe lights.

In 1960 an international committee decided to use the wavelength of one of the colors of krypton-86 to define the meter. A meter was exactly 1,650,762.73 wavelengths of the red-orange spectral line of krypton-86. The meter has since been redefined as 1/299,792,458 of the distance light (in a vacuum) travels in a second.