A fluid is any body that has the property of flow, and lacks rigidity and elasticity, and consequently immediately yields to any force tending to alter its shape and thus adopting the shape of the container that contains it. Fluids can be liquids or gases according to the different intensity of the cohesion forces existing between their molecules.
In liquids, intermolecular forces allow the particles to move freely, although they maintain latent bonds that cause substances in this state to present a constant or fixed volume. When a liquid is poured into a container, the liquid will occupy the partial volume or equal to the volume of the container regardless of the shape of the latter.
Liquids are incompressible because their volume does not decrease when very large forces are exerted on them. Another of their properties is that they exert pressure on the bodies submerged in them or on the walls of the container that contains them. This pressure is called hydrostatic pressure.
Gases, on the other hand, consist of well separated moving particles that collide with each other and try to disperse, in such a way that the gases have no definite shape or volume. And so the container that contains them acquires the shape and they tend to occupy the largest possible volume (they are very expandable).
Gases are compressible; that is, their volume decreases when forces are applied to them. For example, when force is exerted on the plunger of a syringe.
Fluid mechanics is the part of Physics that studies fluids both at rest and in motion, as well as applications and engineering mechanisms that use fluids. Mechanics is divided into fluid statics or hydrostatics, which deals with fluids at rest or in equilibrium; and in fluid dynamics or hydrodynamics, which deals with fluids in motion.
On the other hand, in terms of language, it is said to be fluid to everything that arises easily and is well structured; that is to say, to loose, ordinary, easy and continuous language, without interruptions. For example: Maria has a very fluent German in her qualities.
