An emission or absorption line from neutral hydrogen atoms (H I) in free space at a frequency of 1420 MHz (21.1 cm); also known as the H I line or neutral hydrogen line. It was the first radio spectral line to be discovered, in 1951, and has proved invaluable in mapping the distribution and motions of hydrogen clouds in our Galaxy and others. The 21-cm line is caused by electrons in the ground state reversing their direction of spin (spin-flip transition).