Dispersion measure (DM) is a measure of dispersion (dispersion in the sense of the "smearing" of received signal due to intervening material slowing the EMR such that different wavelengths are slowed by different amounts). It is used in radio astronomy, for transients, e.g., for pulses from pulsars (i.e., the pulsar dispersion measure) and more recently, for fast radio bursts. It is scaled to match a column density of electrons, which along the line of sight would cause that dispersion. Its conventional units are tailored to astronomy: pc cm-3.
t2-t1 DM = ——————————————— kDM (ν2-2-ν1-2)
Given 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