This issue features a glossary of flow measurement terms. Here are some helpful definitions to the most common terms used in flow measurement.
Water has more mass per cubic foot than air, so it's density is higher.
Refers to how much a fluid resists flowing. For example, try stirring a vat of honey. The viscosity of the honey is high, so therefore stirring it will be difficult. A combination of all the factors that influence the flow in a pipe, including pipe diameter, density, velocity and viscosity. With this combination, we can describe flow in a pipe with a single number.