Question

Viscosity is the property of liquids by virtue of which?

Answer 1

Dear Student,

Viscosity is a property of liquid by virtue of which it opposes the relative motion between its different layers.

and yes, the opposing force among layers of the fluid is viscous force.
when the fluid is in a state of rest the fluid experience zero viscosity.
viscosity only exist when there is relative motion among the layers of fluid or w.r.t the Walls of container in which the fluid is present.