Surface Density (mass per surface area or some other quantity per surface area, often signified by Σ) is used in astrophysics at times when volume density might seem to make sense, e.g., the Kennicutt-Schmidt Law.
Measurements of qualities across resolved images of largely-flat things, such as disk galaxies and Circumstellar Disks, (e.g., Luminosity, density of Molecular Clouds, etc.) reveal a surface density distorted by the distance and angle of view. Determining an actual surface density depends upon determining or accommodating these factors and also depends upon the transparency of the object to the quality of interest.
Toomre Q Parameter (Q)