In this article, the concept of body fluid is explained in three aspects: the word meaning of body fluid, the origins of the definitions of the body fluid concept and the connotation and extension of body fluid. Investigating data about the time Hwangjenaegyeong(黃帝內經) was written, the author discovers that the meaning of "Aek(液)" is clear, but there are still questions about the meaning of "Jin(津)". The concept of body fluid derived from observation of life phenomenon and ancient philosophy on the "water". The concept of body fluid should be expressed as that body fluid is a general term for all normal liquids in the body. Within the meridians, as the composition of blood components; outside the meridians, constituting the intrinsic body fluids of various organs and tissues. This is the main part of body fluid, coming from diet, constituting the human body and maintaining human life activities, playing the roles of moistening and nourishing various of organs and tissues of the body. In addition, Interstitial fluid, all kinds of normal liquid secretion and metabolic products, such as sweat, tears, nasal discharge, saliva, slobber, gastric juice, intestinal fluid, urine, joint fluid, latex and so on, both belong to body fluid.