Spectroscopy is the science of studying light after it has been split into its separate colours. It is a field of study, not a machine — the dictionary line calling it 'an instrument' has slipped; the instrument is a spectroscope. Here is the idea in plain words. When a substance burns, glows, or has light shone through it, it does something very particular with the colours: it gives out certain ones and swallows certain others. Every element behaves in its own way, so the pattern of colours works like a fingerprint. Spectroscopy is the practice of reading those fingerprints. It is one of the most powerful tools in science, and its reach is surprising. Astronomers use it to say what a star is made of, and how fast it is moving away from us, without ever going near it. Chemists use it to identify unknown substances. Doctors use it in scanners. Police labs use it on evidence. You will meet the word in physics and chemistry lessons and in any serious article about space. It is a mass noun, so no 'a' and no plural: say 'spectroscopy is used to…', not 'a spectroscopy'. There are many named types — mass spectroscopy, infrared spectroscopy — and those phrases are how you will usually see it.