Abstract: The differences between the concepts of single-,two-and multi-phaseflow and the differences between the concepts of single-, two-and multi-fluidmodel have been studied in this paper.The first three concepts are defined ac-cording to the physical properties of the moving medium,while the second threeare based on the research methods used by authors. Now, there are some con-fusions in using these concepts,such as two-phase flow and multiphase flow,multiphase flow and multifluid model,and so on.The diffusion...