1. The study of spectral lines from different atoms and molecules which is an important part of studying the chemistry that goes on in stars and in interstellar clouds.
2. Any of various techniques for analyzing the energy spectra of beams of particles or for determining mass spectra.

There are many spectroscopic techniques designed for investigating the electromagnetic radiation emitted or absorbed by substances.

Spectroscopy, in various forms, is used for analysis of mixtures, for identifying and determining the structures of chemical compounds, and for investigating energy levels in atoms, ions, and molecules.

In astronomy, it is used for determining the composition of celestial objects and for measuring red shifts.