Let's say that you're at IHOP, and you ordered a nice pancake. Or, if you're one of them, then you ordered a waffle. Despicable.
Anyways, when your delicious meal arrives, you reach over for the syrup container, and spread it over your meal. Every time, the syrup will spill out, and slowly spread out, as shown below.
Most simpletons would describe the liquid as "thick." A less "thick" liquid, such as water, would rapidly spread, ruining your meal.
This property is called, as you can probably guess from the title, viscosity. It is the measure of a fluid's resistance to gradual deformation under stress. High-viscous fluids include honey, syrup, mustard, and ketchup, and low-viscous fluids include water, alcohol, and milk.
The fluid on the left has low viscosity, such as water. The fluid on the right has high viscosity, such as honey.
It's debated that amorphous solids, including glass and many polymers, aren't actually solids, and are actually liquids with very high viscosity.
And ideal fluid, or inviscid fluid is a fluid that has 0 viscosity, and is only observed at very low temperatures in superfluids. This means that it flows without loss in kinetic energy.