Density is defined as mass per unit volume i.e., Density = Mass(m)/Volume(V) Kg/m3 or g/cm3.
Simply Density is the measure of number of particles that are accumulated in certain volume.
Taking an example from Economy,
Population Density Measures the number of people living per unit area.
Formula:
Population Density = No. of people/ Total Area.
Example: If a city has 1,000 people living in a 10 km² area, the population density is 100 people per km².
Likewise, Density in Science and Engineering is No. of particles accumulated in a certain volume.
Best of luck on your journey!
With love,
EngineeringDude.