The difference between the apparent magnitude (m) of an astronomical object, such as a star, and its absolute magnitude (M), used as a distance measurement. Distances can be expressed in distance modulii as $$m-M = 5\log d + 10 = 10-5 log \varpi$$ where \(d\) is the distance in kiloparsec and \(\varpi\) is the parallax in milliarcseconds.

distance modulus

m-M