The well-known 21-cm line (hydrogen line or HI line) is hydrogen's 21.1 cm (microwave) spectral line, which is emitted during a transition between two neutral hydrogen ground states (within the hyperfine structure). The transition is termed a spin-flip transition. The line is a useful sign of the presence of neutral atomic hydrogen. It is a forbidden line, i.e., virtually never created under everyday Earth-bound conditions.
The line is recognizable at a lot of redshifts, and passes through dust clouds better than much electromagnetic radiation. HI regions in galaxies produce 21-cm emission lines used to estimate galaxy mass, and map out velocities through analysis of Doppler shifts. Knowledge of the Milky Way's structure owes much to this analysis. Regions in front of hot objects, i.e., with a continuous spectrum yield 21-cm absorption lines, also useful for analysis.
The neutral hydrogen existing between recombination and the epoch of reionization is presumed to produce 21-cm emission lines, which could reveal much about reionization and times since, including analysis of gravitational lensing to reveal galaxy clusters and Doppler shifts that would reveal voids and is one target of cosmological intensity mapping. The signal is weak and has yet to be detected (as of 2017) but much effort has been put into detecting it (21-cm experiments), such as PAPER and HERA.
The analogous line for deuterium is 91.6 cm (aka 92-cm line). In sources sufficiently bright to see it, this allows determination of a D/1H ratio.