Definition - What does Density mean?
Density is the measurement of quantity or mass per unit of volume in a particular substance. Density is mathematically defined as mass divided by volume. Density differs from element to element on the periodic table and is also affected by environmental conditions including temperature and atmospheric pressure. Density is also effected by other substances which cause chemical reactions.
Safeopedia explains Density
When a material is considered dense, it has molecules that are closer together within a measurement of space or volume. As temperature increases in a substance, it increases pressure, which increases the distance between particles in a substance. This decreases density per unit of volume. This is what leads to melting, as liquids are less dense than solids, and gases are less dense than liquids.