a substance that is fluid at room temperature and pressure
"The mechanic drained the old, dark fluid from the car's engine to replace it with fresh oil."
continuous amorphous matter that tends to flow and to conform to the outline of its container: a liquid or a gas
"The fluid in the broken pipe leaked all over the floor, soaking into the carpet."
Any substance which can flow with relative ease, tends to assume the shape of its container, and obeys Bernoulli's principle; a liquid, gas or plasma.
"The engineer calculated how much pressure would build up as the fluid rushed through the narrow pipe."
In plain English: A fluid is any substance that flows freely, like water or air.
"The water in the glass was clear and fluid."
Usage: As a noun, fluid refers to any substance that flows easily and takes the shape of its container, such as water, air, or oil. Use this term when describing liquids, gases, or plasmas in contrast to solid objects.
Of or relating to fluid.
"The engineer designed a system specifically for the transport of hydraulic fluids."
In plain English: Fluid means something that flows easily and changes shape to fit its container.
"The traffic in the city was fluid during the holiday season."
Usage: Use fluid as an adjective to describe something that flows easily like a liquid, such as water or oil. It can also refer metaphorically to situations that are not fixed and can change quickly, like fluid plans or fluid conversations.
The word fluid comes from the Latin word fluidus, meaning "flowing," which itself derives from the verb fluō ("to flow"). This Latin root traces back to a Proto-Indo-European form associated with swelling or surging, and it is also related to Ancient Greek words for overflowing.