Final answer:
The interstellar medium refers to all the material between stars and is comprised of interstellar matter. It includes giant clouds known as nebulae and is not static, but can change in various properties.
Step-by-step explanation:
Astronomers refer to all the material between stars as interstellar matter; the entire collection of interstellar matter is called the interstellar medium (ISM). Some interstellar material is concentrated into giant clouds, each of which is known as a nebula (plural "nebulae," Latin for "clouds"). The best-known nebulae are the ones that we can see glowing or reflecting visible light. The interstellar medium is not static and can change in density, temperature, and ionization state.