The mass per unit volume of a substance?

The mass of a substance per unit volume is called density. It is a measure of how heavy something is compared to its size.  

Density is most frequently denoted by the lowercase Greek letter ρ 'rho'. 

Density varies widely among materials and may be important in relation to packaging, purity, and buoyancy. The densest known elements under conditions of normal temperature and pressure are osmium and iridium.