Matter can be defined as anything that has mass and occupies space. In its natural form, matter exists as a solid, liquid, gas, or plasma. Matter can be transformed from one state to another via a phase change. Matter is composed of a combination of atoms that form elements and compounds, which are the building blocks of the universe.
At the most abstract level, the universe is made of matter (stuff) and energy (the ability for that stuff to do anything). To make matters even more abstract, the mass-energy equivalence principle states that matter is another form of energy:
Whereis energy, is mass (the amount of matter) and is the speed of light, which gets squared.
For more information on matter, please see UC Davis's chem wiki.