In nuclear magnetic resonance (NMR), the frequency at which a particular nucleus absorbs photons. This is called the resonance frequency and it is a function of both the elemental identity of the nucleus (e.g. nitrogen, carbon, hydrogen) and the local chemical environment around the nucleus. The local chemical environment (things like which other atoms the nucleus is covalently bound to, what kind of bonds are between them, and the nearby presence of charged groups either bonded or not bonded to the nucleus) effects the electron density around the nucleus, and in the NMR's magnetic field these electrons create their own magnetic field opposite in direction to the external magenetic field. The nucleus 'feels' a variable net magnetic field depending on the local electron density, and thus has a variable resonance frequency (see Zeeman effect.)

The term 'chemical shift' was originally intended as an insult. Two different groups of physicists, one at MIT and one at Caltech, were trying to measure an intrinsic property of the hydrogen nucleus by measuring it's resonant frequency in a homemade NMR spectrometer. When they looked at ethanol, they saw three different resonance frequencies, one for each type of proton in the molecule. They wrote a paper describing their data and stated that 'local chemical interactions' obscured the true spin properties of the hydrogen nucleus and this method therefore was 'not promising'.

These 'unpromising' chemical shift differences now form the basis for NMR's use in medicine, chemistry, and biology.