Good day, In my book, the following definition for flow velocity is given: So summarized, the flow velocity at a point in space is the velocity of an infinitesimally small fluid element as it sweeps through that point. But now my question; how is the velocity of an infinitesimally small fluid element defined? We cannot see this fluid element (as opposed to a moving car for example), so how do we define it's velocity? Is it the average velocity of the molecules inside the fluid element? If someone can give me another definition of flow velocity, I would be happy, because the one mentioned in the beginning of my post does not make a lot of sense to me. Thank you in advance.