Question

what is mass density? ​

Answer 1

The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ, although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume: where ρ is the density, m is the mass, and V is the volume

Answer 2

Mass density is the property of any fluid or solid which describe the compactness of any substance by measuring the mass in occupied space by system.

Or

Mass per unit volume is density which tells that how many kg are in metre cube. So that the strength and bonding between molecules can be determined.