Wiktionary
n. (context physics chemistry English) One of any number of ranges of wavelengths of electromagnetic radiation absorbed by a substance.
WordNet
n. a dark band in the spectrum of white light that has been transmitted through a substance that exhibits absorption at selective wavelengths
Wikipedia
According to quantum mechanics, atoms and molecules can only hold certain defined quantities of energy, or exist in specific states. When such quanta of electromagnetic radiation are emitted or absorbed by an atom or molecule, the energy of the radiation changes the state of the atom or molecule from an initial state to a final state. An absorption band is a range of wavelengths, frequencies or energies in the electromagnetic spectrum which are characteristic of a particular transition from initial to final state in a substance.
Usage examples of "absorption band".
When Galileo photographed the earth, it found unmistakable evidence of a sharp absorption band painting the continents: some substance was soaking up radiation at wavelengths around 0.
Now, notice also the sharp absorption band at the far red end of the visible spectrum—.