What is Chemical shift, shielding, equivalent protons?

The chemical shift of a proton is $\delta = (\nu - \nu_{\text{ref}})/\nu_0 \times 10^6$, in $\mathrm{ppm}$, where $\nu$ is its resonance frequency, $\nu_{\text{ref}}$ that of the protons of tetramethylsilane and $\nu_0$ the operating frequency of the spectrometer. The reduction of the field at a nucleus by its electrons is its shielding: electron-withdrawing neighbours (O, halogens, C=O) deshield a proton and raise its $\delta$. Equivalent protons are protons exchanged by a symmetry of the molecule or by a fast rotation; they have the same shift.