Final answer:
Density is defined as the mass or weight of a substance in a given amount of space, described as the mass per unit volume. It varies depending on the substance's volume and mass, and it is crucial for understanding material properties.
Step-by-step explanation:
The mass or weight of a substance in a given amount of space is defined as density. This concept is central to understanding the physical properties of materials, and is expressed mathematically as density = mass/volume. The units of density are generally a combination of mass units (such as kilograms) and volume units (like cubic meters).
To elaborate, if we have two objects with the same mass but different volumes, they will have different densities. For example, a kilogram of cotton balls has much more volume and thus lower density compared to a kilogram of gold, which has a smaller volume and higher density.
It's important to note that density can be affected by environmental factors, such as temperature and pressure, which can cause substances to expand or contract and thus change their volume.