Electron density is a representation of the probability of finding an electron in a specific location around an atom or molecule. In general, the electron is more likely to be found in regions with high electron density. However, due to the uncertainty principle, it's not possible to identify the exact location of an electron at any instant in time. For a system with a single electron, electron density is proportion to the square of its wavefunction. X-ray diffraction crystallography is one technique used to measure electron density.
When the concept is applied to free radicals, it is called spin density. This is the total electron density of one spin minus the electron density of electrons with the other spin. Neutron diffraction is used to map spin density.