The electric form factor is the

Fourier transform A Fourier transform (FT) is a mathematical transform that decomposes functions into frequency components, which are represented by the output of the transform as a function of frequency. Most commonly functions of time or space are transformed, ...

of electric charge distribution in a nucleon. Nucleons (protons and neutrons) are made of up and down quarks which have charges associated with them (2/3 & -1/3, respectively). The study of Form Factors falls within the regime of Perturbative QCD. The idea originated from young William Thomson

See also