According to Dictionary.com, matter has three definitions relating to science: the substance that composes physical objects, general corporeal or physical substance and that which occupies space. Matter has other definitions not relating to science as well.
Matter doesn't have a universally accepted definition because some types of matter do not have mass unless they are in motion. The pre-20th Century definition of matter states that it is anything composed of atoms, though scientists claim that too many problems arise from this definition. As a result of the difficulty defining matter, it is not a fundamental concept in the field of physics.