The number of free electrons per unit volume of space. Typical values range from less than 10−4 electrons/cm3 in intergalactic space to 0.03 electrons/cm3 in the disk of our Galaxy, 104 electrons/cm3 in H II regions, and 108 electrons/cm3 in stellar winds. Electron density in the Galaxy can be mapped from the dispersion measure of pulsars. Conversely, for a part of the Galaxy in which the electron density is well-known, the distance of a radio source may be derived from its dispersion measure. Electron density is directly related to the amount of ionized hydrogen and is a useful probe of conditions in interstellar space.