Hello! I would say that "liquid" describes the consistency of something - a substance that flows freely but is of constant volume, having a consistency like that of water or oil.. For example, "The fluid drained through the sieve leaving behind the solid material that was too big to fit through the tiny openings." and "fluid" is a substance that has no fixed shape and yields easily to external pressure; a gas or (especially) a liquid; or able to flow easily. For example; "The fluid movements of the dancers were fascinating." I hope that helps!

Liquids are fluids but not all fluids are not necessarily liquids. Liquids and gasses are fluids because they take the shape of their container, they move and they are affected by the pressure and volume of a certain place or container.

A fluid could be a gas, a liquid or, arguably, a solid under heating or pressure which is just starting to flow. As adjectives, "fluid" suggests smooth, continuous movement or something that easily changes shape, whereas "liquid" suggests lack of structure - there is no fixed shape. In my opinion, of course ! https://www.lexico.com/definition/liquid https://www.lexico.com/definition/fluid https://en.wikipedia.org/wiki/Fluid

You could use the fluid term in the field of mechanical Engineering so many times but if you want to distinguish elements according to their features then it is supposed to be varied according to liquid or solid or gas

There are 3 states of matter (i.e. things). There are solids, liquids and gas. Plasma is the 4th state of matter, but not relevant here. Water can exist in all 3 states: as a solid it is called ice, snow, hail, etc. As a liquid it is called water. As a gas it is called steam or water vapour. That is what a liquid is. A fluid is something that flows (from Latin /Spanish "fluir" to flow). Liquids can flow, and so can gases. Gas can flow in pipes, as wind, etc and it can fill a space, just like a liquid does. We also have the phrase 'a fluid situation" which means that it is not yet certain, it is still 'flowing' and not one way or another. As well as the phrases and situations in Zofia's answer.