A Spectral Line is a dark or light line in a Continuous Spectrum. For example, the Magnitude of different Wavelengths of light from something may generally be described curve, i.e., nearby wavelengths have nearly the same magnitude. A line would be an exception to this, i.e., a "spike" indicating a wavelength at which light has a much higher magnitude than nearby wavelengths, or a dip where a wavelength has a much lower magnitude than nearby wavelengths.
Light lines are generally caused by Emissions at specific wavelengths (Emission Lines), and dark lines are caused by Absorption at specific wavelengths (Absorption Lines). Lines are related to chemical makeup of the light source or absorber, indicating changes of energy level of electrons, which have characteristic values based upon chemistry.
Characteristics of the details of lines can show other characteristics of the light source or intervening absorber such as velocity, Turbulence, etc.
Among the widely used spectral lines are the Hydrogen series of lines, e.g.,
Atomic spectral lines are sometimes classified by Element and ionization, specified by the chemical symbol followed by a Roman numeral, I meaning neutral, II meaning singly ionized positive, III meaning doubly ionized, etc.
The term Line Broadening refers to the mechanisms that create the shape of a line, e.g., in Electromagnetic Radiation from a star.
Spectral-Line Mapping refers to imaging or surveying a portion of the sky for the presence of particular spectral lines, often noting their Redshift, to determine their Radial Velocity. Its uses include investigations of Accretion Disks, Jets, Molecular Clouds, the entire Milky Way, and other galaxies.