A fictitious temperature that describes the average spin of neutral hydrogen (H I) atoms in a gas cloud. H I has two energy states: an upper state in which the spins of the electron and proton are parallel, and a lower state in which the spins are opposite (antiparallel). The spin temperature of an H I region is the temperature that would produce the observed ratio of parallel to antiparallel spins if the H I gas were in thermal equilibrium. The transition between the two spin states of the H I atom produces the important radio spectral line at 21 cm wavelength.