By considering the concept of point sources, such as point charge, mass point, and heat point source, we give a unified definition of these point sources with three conditions to judge what kind of distributed source in a small region can be taken as a point source by applying Sternberg's concept on concentrated forces. Comparing it with the traditional qualitative understanding of this concept, the definition gives a new condition with a very simple form. Counter-examples demonstrate that the traditional understanding of the point source is not sufficient and the new requirement should not be omitted.