flu·id / ˈfloōid/ • n. a substance that has no fixed shape and yields easily to external pressure; a gas or (esp.) a liquid: we all need several glasses of fluid a day | a cleaning fluid. • adj. (of a substance) able to flow easily: the paint is more fluid than tube watercolors a fluid medium. ∎  not settled or stable; likely or able to change: our plans are still fluid the fluid political situation of the 1930s. ∎  smoothly elegant or graceful: her movements were fluid and beautiful to watch. ∎  (of a clutch or coupling) using a liquid to transmit power.DERIVATIVES: flu·id·ic / ˈfloōˈidik/ adj.flu·id·i·ty / floōˈidətē/ n.flu·id·ly adv.

fluid Any substance that has no fixed state and is able to flow. Of the three common states of matter, gas and liquid are considered fluid, while a solid is not.

fluid adj. XV; sb. XVII. — (O)F. fluide or L. fluidus, f. fluere flow; see -ID1.
Hence fluidity XVII.