The word "fluid" refers to something that can easily change or move smoothly. It can describe both liquids that flow easily like water, as well as ideas or situations that can change or adapt easily. Full definition
Proper levels of fluids in your vehicle allow it to function smoothly, giving you a better ride and increasing the life of your car. (indyautoblog.com)
This is a general property of rising and falling bodies of fluid in a rotating system. (discovermagazine.com)