The fractional variation of the resonance frequency of a nucleus in nuclear magnetic resonance (NMR) spectroscopy in consequence of its magnetic environment. The chemical shift of a nucleus, \(\delta \), is expressed as a ratio involving its frequency, \(\nu _{\mathrm{cpd}}\), relative to that of a standard, \(\nu _{\mathrm{ref}}\), and defined as: \[\delta = \frac{\nu _{\rm{cpd}}- \nu _{\rm{ref}}}{\nu _{ref}}\] \(\delta \)-values are normally expressed in \(\mathrm{ppm}\). For $\ce{^{1}H}$ and $\ce{^{13}C}$ NMR the reference signal is usually that of tetramethylsilane (TMS), strictly speaking in dilute solution in $\ce{CDCl3}$. Other references are used in the older literature and for other solvents, such as $\ce{D2O}$. Resonance lines to high frequency from the TMS signal have positive, and resonance lines to low frequency from TMS have negative, \(\delta \)-values (arising from relative deshielding and shielding respectively). The archaic terms 'downfield' and 'upfield' should no longer be used. For nuclei other than $\ce{^{1}H}$, chemical shifts are expressed either in the same manner relative to an agreed substance containing the relevant nucleus or relative to the $\ce{^{1}H}$ resonance of TMS as \(\mathit{\Xi}\) values, defined in the references below.