Most fluids are liquids or gases. Examples include air and water. A fluid is a material that flows or continuously deforms under a shear (tangential stress). In other words, a fluid has zero shear modulus. Liquids, gases, and plasma are fluids. However, some solids behave as fluids as well.
A fluid in medicine or biology refers to any liquid constituent of the body (body fluid), [6][7] whereas "liquid" is not used in this sense. Sometimes liquids given for fluid replacement, either by drinking or by injection, are also called fluids [8] (e.g. "drink plenty of fluids").