Mass

Mass is a measure of the amount of stuff in something. On earth, this is generally the same as weight. The more something weighs, the more mass it has. However, mass and weight are not exactly the same. Weight requires gravity—it is actually a measurement of the effect of gravity on something, which of course depends on the amount of mass. But even in space, things have mass. In addition, in chemistry you will study gases which don’t seem to have any weight, yet are matter and do have mass.

A better test for whether something has mass is that you can feel mass when it hits you. This definition works in space, since a weightless hammer would still hurt if it hit you in the head. More importantly, the definition works for gases here on Earth. Just ask anyone who has ever been through a hurricane, felt the wind and seen destruction that moving air can cause.

This also means that anything which you can’t feel hit you, doesn’t have mass, and therefore isn’t matter. Light, for instance is massless. This can be easily shown. Put on a blindfold and then have someone point a flashlight at your fingertips (the most sensitive part of the hand). Have them move the light beam on and off of your hand. You will not be able to feel when the light is there and when it is not. Thus, light does not have mass and is not matter. When we "feel" sunlight hit us, we are really feeling the energy from the light make our skin warmer. Thus is it the temperature change that we feel, not the impact of the light.

Having mass is half of the definition of matter. The other half is that matter has volume.