The definition of inbound is something that moves inward. An example of inbound is the freeway that goes toward the city center. 1  traveling or going inward.  Basketball inbounds.
Mass is a measurement of how much matter is in an object. Mass is a combination of the total number of atoms, the density of the atoms, and the type of atoms in an object. How to Measure Mass.
Matter is that substance that can occupies space and has mass. For example gas,liquid etc. are mass like my friends said. But photons which are fundamental particles of light occupies space but dont have mass,meaning they are not matter. Hope this helps.