The term plasma has two major definitions in science. In biology, it refers to the clear, straw-colored liquid portion of blood. In physics, it refers to a state of matter in which atoms are completely ionized. By this definition, a plasma consists entirely of separate positive and negative ions. Plasmas of this kind exist only at high temperatures. The study of plasma is extremely important in research on nuclear fusion. One of the most difficult problems in that research is to find ways of physically containing a plasma in which fusion reactions are occurring.