1.In chemistry electron density is the measure of the probability of an electron being present at a specific location. According to quantum mechanics, due to the uncertainty principle on an atomic scale the exact location of an electron cannot be predicted, only the probability of its being at a given position; therefore electrons in atoms and molecules act as if they are "smeared out" in space. The electron density at any point is proportional to the square magnitude of the wavefunction.
2.Electron density maps are the end results of an X-ray structure determination. The maps combine the structural model and the experimentally collected data to represent how well the model fits the data.
There are two common types of electron density maps used by researchers, the 2Fo-Fc map and Fo-Fc map. The Fo-Fc map, known also as a difference or omit map, is used to show what has been overfit or not accounted for by the model, while the 2Fo-Fc map will include the Fo-Fc map and electron density around the model.
These two maps are then used to correct the model when possible. Even in the best quality structures, there are areas of poor electron density, which may represent sections of the model that exist in multiple conformations. This can be seen in long side chains or surface loops of the model.
More information: http://reference.iucr.org/dictionary/Electron_density_map