Illustrated Glossary of Organic Chemistry

Electron density map: In x-ray crystallography, an interpretation of the diffraction pattern as a plot of electron density versus position in space. Can be thought of as a two-dimensional slice through a molecule's electron cloud. Often plotted as a topographical map, where each atom's electron density is a mountain peak.

An electron density map.   
Electron density map with bonds and atomic
positions (except hydrogens) plotted.