21-centimeter line

21-centimeter line

[¦twen·tē¦wən ′sen·tə‚mēd·ər ‚līn]
(spectroscopy)
A radio-frequency spectral line of neutral atomic hydrogen at a wavelength of approximately 21 centimeters and a frequency of approximately 1420 megahertz, that results from hyperfine transitions between states in which the spins of the electron and proton are parallel and antiparallel.