The modern definition of matter is not universally agreed upon, but it generally refers to substances that occupy space and have mass, including particles like atoms and molecules. Electrons are typically classified as matter, despite their wave-like behavior and having a rest mass, which leads to discussions about the nature of particles and mass. Some argue that only particles with non-zero rest mass should be considered matter, while others include all particles, regardless of mass. The ambiguity surrounding the term "matter" arises from its varying definitions across different scientific fields. Ultimately, in theoretical physics, matter is often defined as anything that possesses mass.