Sometimes Mac Pro Dictionary defines it better than I can:
The state of being thick, sticky, and semifluid in consistency, due to internal friction.
• a quantity expressing the magnitude of such friction, as measured by the force per unit area resisting a flow in which parallel layers unit distance apart have unit speed relative to one another.
If your coffee, milk, juice, soda, alcohol if viscous, THROW IT OUT NOW! If it’s olive oil, don’t worry, it’s supposed to be that way.
Note: Greek’s the best.