Generally, spectroscopy that relies upon measuring the radiation absorbed by a sample as it is excited to higher energy levels.
Absorption spectroscopy is a technique for chemical analysis that involves measuring how much electromagnetic radiation the sample absorbs at various wavelengths. A profile of absorbance versus wavelength (called an absorption spectrum) is often used as a "fingerprint" for a particular chemical compound.
Compare with absorption spectrum. A technique for determining the concentration and structure of a substance by measuring the amound of electromagnetic radiation the sample absorbs at various wavelengths.