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Electron density is the measure of the probability of an electron being present at a specific location.
In atoms, regions of electron density are usually found around the atom, and its bonds. In delocalised or conjugated systems, such as phenol and benzene (and compounds such as haemoglobin and chlorophyll) the electron density covers an entire region, i.e., in benzene they are found above and below the planar ring. This is sometimes shown diagramatically as a series of alternating single and double bonds. In the case of phenol and benzene, a circle inside a hexagon shows the delocalised nature of the compound. This is shown below:
In compounds with multiple ring systems which are interconnected, this is no longer accurate, so alternating single and double bonds are used. In compounds such as chlorophyll and phenol, some diagrams show a dotted or dashed line to represent the delocalisation of areas where the electron density is higher next to the single bonds (e.g., the white line in the diagram on Chlorophylls and Carotenoids (http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chlorophyll.html)). Conjugated systems can sometimes represent regions where electromagnetic radiation is absorbed at different wavelengths resulting in compounds appearing coloured. In polymers, these areas are known as chromophores.
Electron densities are sometimes probed with X-ray diffraction scans, where X-rays of a suitable wavelength are targeted towards a sample and measurements are made over time to represent, probabilistically, where electrons can be found. Quantum electrodynamics and some branches of quantum theory also study and analyse electron superposition and other phenomenon. Quantum tunnelling and quantum entanglement are interesting areas involving electrons (or photons). High speed electrons are often used in transmission electron micrography (or microscopy, TEM) and deep inelastic scattering, as well as many other high-speed particle experiments involve electrons.
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