What is viscosity in terms of chemistry?
More or less similar…I will try to put brief concepts of viscosity in terms of pharmaceutics which is somewhat another branch of chemistry, where it is very much applicable in drug design.



Viscosity is an expression of the resistance to flow of a system under an applied stress. The more viscous a liquid is, the greater is the applied force required to make it flow at a particular rate.


  Viscosity is defined in terms of the force required to move one plane
  surface past another under specified conditions when the space between
  is filled by the liquid in question. More simply, it can be considered
  as a relative property, with water as the reference material.


Viscosity implies that the liquid flows even under the smallest stress and does not return to its original shape or form once the stress is removed.

Viscous deformation, i.e. viscous flow, occurs in accordance with Newton’s law,

$\ce{σ=ηỳ}$


  where the applied stress σ results in flow with a velocity gradient ỳ
  or rate of shear. The proportionality constant η is termed viscosity,
  while its reciprocal is called fluidity.


Viscosity has also been
 described as the internal friction in the fluid as it corresponds to the resistance of the fluid to the relative motion of adjacent layers
 of liquid.

The viscosity of simple liquids (i.e, pure liquids consisting of small molecules and solutions where solute and solvent are small molecules) depends only on composition, temperature, and pressure. It increases moderately with increasing pressure and markedly with decreasing temperature.

There is quite a lot to discuss on viscosity in depth but these are just shallow concepts to understanding the term.



References


Martin’s Physical Pharmacy and Pharmaceutical Sciences : Rheology
Remington: Essentials of Pharmaceutics: Rheology
Ansel’s Pharmaceutical Dosage Forms and Drug Delivery Systems : Special Solutions and Suspensions
Aulton ’s Pharmaceutics The Design and Manufacture of Medicines 4th edViscosity Rheology, and flow of fluids