hyperfine structure

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of the spectral lines formed from the splitting of broader spectral lines as a result of the interaction between the magnetic moments of electrons and atomic nuclei.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics a closely spaced structure of lines in the spectrum of an atom or molecule due to nuclear isotopic effects and/or to nuclear spin