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I think all the liquids are viscous. So I was wondering what is meant by a viscous liquid as it is written in the first line of page on Oil at Wikipedia.

An oil is any neutral, nonpolar chemical substance that is a viscous liquid at ambient temperatures and is both hydrophobic (immiscible with water, literally "water fearing") and lipophilic (miscible with other oils, literally "fat loving"). ...

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    $\begingroup$ Yes, fluids are viscous (well, superfluids...). They will have a viscosity. The liquid we are most familiar with, water, is viscous. But in general use, if something is viscous, it is more viscous than water, and usually noticeably more viscous. I"m not sure there is an official viscosity where things are labeled 'viscous'. $\endgroup$
    – Jon Custer
    Commented Dec 18, 2015 at 14:09

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This is somewhat an elementary discussion, but viscosity is typically measured in some sort of viscometer. A typical one is the Oswald viscometer shown below.

enter image description here

Fluid is poured into the apparatus through tube at $f$ to fill neck and bulb at the bottom. The fluid is then sucked up above line $c$. The fluid is then released and drains down the tube to line $c$ at which time the timing for the flow rate starts. When the fluid falls to the line at $d$ is reached then a flow rate can be measured. By using samples of known viscosity, the flow time can be calibrated to the viscosity of the sample.

Now such measurements are in units of pascal-seconds. For reference a short list will be provided.

     Substance           mPa-s
     acetone                0.0306
     water                  0.0894  
     sulfuric acid          0.242
     olive oil             81.
     motor oil SAE 40     319.
     glycerol           1,200
     corn syrup         1,380.6
     pitch              2.3×10^8

Wikipedia also lists tables for Viscosity of selected substances

Now let's defined three ranges of viscosity:

  • Low Viscosity - 0.1 mPa-s >= sample
  • Medium Viscosity - 0.1 mPa-s <= sample <=1.0 mPa-s
  • High Viscosity - sample > 1.0 mPa-s

Of course the "exact" ranges are arbitrary. So a viscous liquid is a High Viscosity liquid on our scale.

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A viscous fluid is simply defined as a fluid that resists flow due to internal friction for example if you poured oil into a cup with a hole in it ,it certainly would flow less faster than the same quantity of water.

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Generally viscosity is an ability to resist flow. More viscous liquid will flow slower. Oil and honey are viscous liquids i.e. liquids with high viscosity.

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