The measure of how much matter is given space is called density. Density is defined as the mass of an object per unit volume. It is a scalar quantity and is expressed in units such as kilograms per cubic meter (kg/m³), grams per cubic centimeter (g/cm³), or pounds per cubic foot (lb/ft³).

Density is an important property of matter as it provides information about how tightly packed the particles of an object are. Objects with higher density are more compact, while objects with lower density are less compact. For example, lead is a very dense metal, while air is a very low-density fluid.

Density can be used to determine the mass of an object if its volume is known, or to determine the volume of an object if its mass is known. It is also used to compare the relative densities of different objects and to determine the composition of an object.