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adj. Extremely fine, especially of the hyperfine structure in the spectra of atoms and molecules.

adj. extremely fine or thin, as in a spectral line split into two or more components

"The magnetic field separates these levels into two "hyperfine" states."

thin   

spin­orbit    quadrupole    magnetostatic    energy­momentum    dipolar    field-induced    non-covalent    energy-dispersive    quadrupolar    pharmacokinetic