Ram pressure, Pr, is the effective pressure of a moving fluid above its thermal or static pressure. In astronomy, it is used to calculate the interaction of stellar winds with planets and to quantify the effects of ram pressure stripping. On airplanes and in wind tunnels, it is measured with a pitot-static tube, an inexpensive device with no moving parts that was invented in 1732. Up through the mid 1960s, across both astronomy and engineering the ram pressure of a moving gas and its momentum flux, ρu2, where and u are the upstream mass density and flow speed, were properly treated as related but distinct quantities. This relationship may be expressed as Pr = Sp ρu2, where Sp is the dimensionless Spreiter number, which ranges between 0:5 and 0:88 for a monatomic gas, depending on the upstream Mach number, Ma. Unfortunately, by the early 1970s, in astronomy ram pressure was defined to be the momentum flux and Sp was fixed to be unity and forgotten as a parameter. In this talk we will review the determination of Sp for subsonic and supersonic flow.