What is meant by viscous liquid?
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.



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 &lt;= sample &lt;=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.