In NMR, nuclei can be classified as isochronous or anisochronous. “Where diastereotopic protons show the same chemical shift, they are said to be accidentally equivalent or isochronous, and where they have different chemical shifts the protons are described as anisochronous.” Stereochemistry by David G. Morris, Royal Society of Chemistry (Great Britain) Published by Royal Society of Chemistry, 2001.
The 1H NMR spectrum is shown for geminal protons, coloured in red, within a ring system. The spectrum illustrates protons from a methylene group (CH2) with different chemical shifts that are coupled to each other. The protons are termed as diastereotopic and anisochronous.