The mass of an object is a fundamental property of the object; a numerical measure of its inertia; a fundamental measure of the amount of matter in the object. Definitions of mass often seem circular because it is such a fundamental quantity that it is hard to define in terms of something else. All mechanical quantities can be defined in terms of mass, length, and time. The usual symbol for mass is m and its SI unit is the kilogram. 

    Typically mass is referred as the "amount of particles in a volume".  Those particles are normally referring to Atoms or molecules, but there are a whole list of other particles that are not atoms or molecules. So the chemistry definition (the wrong definition) is the number atoms in a volume

What is really "funny" is that this definition isn't good enough for most modern physicists.  As humans we have spent billions of dollars and man hours to "find" a better definition (watch the last  two videos to get an idea what we think mass is)