A fluid is any substance that flows.

What is a Fluid?

A fluid is any substance that flows, including liquids, gasses, plasmas, and to some extent, even plastic solids like Silly Putty. More specifically, a fluid is any substance that continually deforms under applied shear stress over any period of time; it’s a substance that has viscosity and is not a true solid.[1]

It works like this:

  • Fluid: Any substance that flows.
  • Viscosity: The thickness of a fluid. The higher the viscosity, the greater its resistance to flow.

Non-classical states aside, all matter exists as one of four classical states:[2]

  • Solid (definite shape and volume).
  • Liquid (definite volume but not shape).
  • Gas (no definite shape or volume).
  • Plasma (no definite shape or volume, responds to electromagnetic force).

Anything that is not a “true solid,” and thus isn’t a definite shape (as measured within a given timeframe), has viscosity and is a type of fluid.

What is Viscosity?

TIP: Glass is an amorphous solid (and doesn’t flow despite some claims), see other non-classical states of matter (many of which are types of fluids).

FACT: The air we breath is a fluid, so is resin, and so is fire. Honey is more viscous (more resistant to flow) than water. In some ways (see below) silly putty is both a fluid and a solid. Granite can flow over time. Physics is strange.

Fluid in Physics Vs. Fluid in Common Language.

In common language the term fluid is often used as synonym for “liquid,” but in physics, a fluid is any state of matter in which the component particles (generally molecules) can move past one another and cannot resist any shear force applied to them.

When discussing physics, anything that is not a pure solid is a fluid or has fluid-like properties, namely “viscosity.”

Lecture 1 part 1 What is a fluid. This is going to blow your mind, but there are a lack of short snazzy videos online discussing fluid mechanics. What this video lacks in snaz, it makes up for in smart.

What is Viscosity?

Viscosity is a measure of a fluid’s resistance to flow; A measure of its “thickness.” The internal friction of a moving fluid resulting from its molecular makeup is the basis of resistance.[3]

A fluid that has no resistance to shear stress is known as an ideal or inviscid fluid (a “true fluid”).[4]

FACT: In general, a fluid’s viscosity decreases as the fluid is heated and increases as the fluid is cooled.

FACT: Under normal circumstances (such as temperatures not approaching true zero), all fluids have positive viscosity; they are viscous. At close to absolute zero, a second liquid state described as superfluid because it has zero viscosity or infinite fluidity; i.e., flowing without friction.[5]

High Viscosity Vs. Low Viscosity

  • High viscosity means thick.
  • Low viscosity means thin.

The higher the viscosity of the liquid, the more resistance it has to flowing, the lower the viscosity, the easier it flows.

Honey has a high viscosity; water has a low viscosity. A fluid with a relatively high viscosity, such as pitch, may appear to be solid unless you observe it over long periods of time.

When a substance doesn’t flow at all, when it has no viscosity, it is a “true solid.”

The Sci Guys: Science at Home – SE2 – EP7: Viscosity of Liquids

Elasticity Vs. Viscosity and Viscoelastic Fluids

The elasticity of a solid (the ability of a body to resist a deforming when stress is applied) should not be confused with viscosity. The viscosity of a solid should not be confused with elasticity.

Fluids will briefly react like solids when under sudden stress. Think of high diving into water. Many solids can flow like liquids, albeit very slowly, even under arbitrarily small stress such as rock formations over time.

Things that have elasticity and flow, an example being plastic solids like Silly Putty, are best described as being “viscoelastic.” When the behavior of Silly Putty is observed over time, we can observe its viscosity. When it is observed over a short time it behaves like a solid and has elasticity.[6]

Oobleck and Non-Newtonian Fluids: Crash Course Kids #46.1.

Exploring Fluid Dynamics

The study of fluids is called fluid mechanics. The study of “flow” is called fluid dynamics. The units of pressure used in fluid dynamics are called Pascals, named after French polymath Blaise Pascal. Check out the video below for an introduction to fluid dynamics.[7]

20. Fluid Dynamics and Statics and Bernoulli’s Equation


In common language “fluid” means “liquid”, but in physics “a fluid” is any substance that flows.


See Fluids in Physics: Definition and Characteristics from Study.com for more information.


  1. Fluid
  2. Chapter 4: Viscosity: Thick or Thin?
  3. What is viscosity?
  4. Viscosity
  5. Superfluid
  6. viscoelastic
  7. Pascal (unit)

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