Anything that has weight and shape has mass. In fact, mass is the quality that gives things weight anywhere there's gravity.
If you think about mass being about weight and shape, you can see how many of its meanings are related. If something is very big we say it has great mass — or is massive. If we talk about a large group of people, we talk about a mass demonstration or a mass of demonstrators. We refer to poor or common people as the masses — there are a lot of them. Doctors refer to tumors inside our bodies as masses.
Definitions of mass
noun
the property of a body that causes it to have weight in a gravitational field
(physics) the mass of a body in motion relative to the observer: it is equal to the rest mass multiplied by a factor that is greater than 1 and that increases as the magnitude of the velocity increases
(physics) the mass of a body as determined by the second law of motion from the acceleration of the body when it is subjected to a force that is not due to gravity