Physics is a subject that studies the behavior of matter and energy. It is a branch of science that deals with the structure of matter, its motion, and its interactions with energy and forces. The two crossword solutions for the term 'physics subject' are 'ION' and 'ATOM'. An ION is an ATOM or molecule that has gained or lost one or more electrons, giving it a net positive or negative electrical charge. An ATOM is the smallest unit of matter that retains its chemical properties. It consists of a nucleus surrounded by electrons.