Matter measured as mass per unit volume (eg kilogram per cubic meter).
The quantity of being dense, close, or compact. Its numerical value varies with the units used. A measure of how heavy a solid, liquid or gas is for its size. Density is expressed in terms of weight per unit volume.
Plays a role in the movement of contaminants. Density measurements are a useful means for identifying substances. Density plays an important role in the flow of fluids through the subsurface.
The oil is less dense than the water, so a visible boundary is present between the two. The superball is less dense than water, but more dense than oil, so it sinks to the bottom of the oil layer, yet floats on the top of the water layer.
Source: demo.physics.uiuc.edu