The time for which a particle at a temperature suitable for thermonuclear fusion is held in the fusion device. It is denoted by the Greek letter tau. Confinement time, along with temperature and density, is an important parameter of the Lawson criterion indicating whether thermonuclear fusion can take place successfully in plasma. In magnetic confinement devices, the pulse length can be much longer than the confinement time, which determines how long we can keep an appropriately heated particle in the reaction space before it escapes and has to be replaced by a new particle that needs to be heated.