Matter should not be confused with <answer>mass</answer>, as the two are not quite the same in modern physics. For example, <answer>mass</answer> is a conserved quantity, which means that its value is unchanging through time, within closed systems. However, matter is not conserved in such systems, although this is not obvious in ordinary conditions on Earth, where matter is approximately conserved. Still, special relativity shows that matter may disappear by conversion into energy, even inside closed systems, and it can also be created from energy, within such systems. However, because <answer>mass</answer> (like energy) can neither be created nor destroyed, the quantity of <answer>mass</answer> and the quantity of energy remain the same during a transformation of matter (which represents a certain amount of energy) into non-material (i.e., non-matter) energy. This is also true in the reverse transformation of energy into matter.
What similar term is often used interchangeably with matter, although the two differ slightly in modern physics?
mass