For example: A rock is obviously more dense than a crumpled piece of paper of the same size.
A Styrofoam cup is less dense than a ceramic cup.
Density may also refer to how closely "packed" or "crowded" the material appears to be - again refer to the Styrofoam vs. ceramic cup.
The formal definition of density is mass per unit volume. Usually the density is expressed in grams per mL or cc. Mathematically a "per" statement is translated as a division. cc is a cubic centimeter and is equal to a mL Therefore,

Where:
m = mass (kg)
V = Volume (m3)
ρ = Density (kg/cm3)
Example: Calculate the density in g/mL of aluminum if a 50 mL block weighs 135 g.
Solution:
Apply the definition:
Density = 135 : 2,70 = 50 g/mL