Dispersion measure (DM) is a measure of the dispersion of a signal, i.e., the phenomenon of the duration of a short signal growing longer as it travels. Electromagnetic radiation is slowed by the material it is passing through, generally, the longer the wavelength, the more it is slowed, so the shorter wavelengths of the signal are received earlier than the lower wavelengths that were sent at the same time, and dispersion measure is a measure of to what degree additional wavelength delays the signal. It is used in radio astronomy for transients consisting of short pulses, such as those of pulsars (i.e., the pulsar dispersion measure) and more recently, it is used for fast radio bursts. It is scaled to match a column density of electrons that would cause the dispersion, using conventional units tailored to astronomy: pc cm-3.
t2-t1 DM = ——————————————— kDM (ν2-2-ν1-2)
Coupled with a typical interstellar medium electron density, the dispersion measure yields an estimate of the distance to the source. Such an estimate far exceeding the span of the Milky Way is evidence that a source is extragalactic. In such cases, a model of the sources of dispersion can be attempted:
DMobserved = DMMW + DMintergalactic medium + DMhost galaxy