Refer to Section 11 for a complete list of symbols used in the following definitions. Some viscosity terms relating to oscillatory measurements are defined both here and more explicitly in section 8.

**apparent viscosity,** _{app} The value of viscosity evaluated at
some nominal average value of the
*shear rate*. The apparent viscosity applies, for instance, in the
*capillary method*, where a range of shear rates are employed.

**coefficient of fluidity [fluidity]**,
Reciprocal of the
*coefficient of viscosity*.

= 1 /

**coefficient of viscosity
[viscosity]**,
The ratio of
*shear stress* to
*shear rate* under simple
*steady shear*. The abbreviated form 'viscosity' is used most often in practice, and is frequently employed without discriminating between Newtonian and non-Newtonian behavior. When the quotient is independent of shear rate (i.e., follows the
*Newtonian* model), it is considered a material constant. When the quotient is dependent on shear rate (i.e.,
*non-Newtonian*), it should be referred to as the **non-Newtonian viscosity**.

= /

**complex viscosity**, ^{* } The frequency-dependent viscosity function
determined during
*forced harmonic oscillation* of shear stress; contains both real and imaginary parts.

**differential viscosity**, _{diff} The derivative of shear stress with respect to shear rate.

_{diff}
= /

**dynamic viscosity**,
^{ '}
The ratio of the
*loss modulus* to the angular frequency, determined during
*forced harmonic oscillation* (dynamic) measurements. The real part of the
*complex viscosity*.

^{'
} = G^{"} /

**infinite shear viscosity**,
The high
*shear rate* limiting value of viscosity. Often associated with the
second Newtonian region in
*pseudoplastic* fluids.

**inherent viscosity [logarithmic viscosity]**, _{ inh}
A natural logarithmic function of the
*relative viscosity* reduced by the solute concentration.

**intrinsic viscosity [limiting viscosity
number]**, [ ] The zero
concentration
limiting value of the
*reduced specific viscosity*. A characteristic
function for the single molecule in solution. [ ] is equivalent to the effective hydrodynamic specific volume for the solute.

**kinematic viscosity**,
The ratio of the viscosity of a fluid to its density.

= _{s} /

**out-of-phase viscosity,
**^{''}
The ratio of the
*storage modulus* to the angular frequency, determined during
*forced harmonic oscillation* (dynamic) measurements. The imaginary
part of the
*complex viscosity*.

^{''} = G^{'
}/

**plastic viscosity**,
_{ pl}
For a
*Bingham* model, the excess of the
*shear stress* over the
*yield stress* divided by the
*shear rate*, and equal to the
*differential viscosity*. For
*non-ideal Bingham* materials, the differential viscosity determined
in the high-shear limiting, linear portion of the flow curve; associated with
*viscoplastic* fluids.

**reduced viscosity [viscosity number]**,
_{ red}
The ratio of any viscosity function to the concentration, c, of the solute or particulate phase. By normalizing viscosity in this way, concentration effects become more apparent. For instance, the reduced
*specific viscosity* is:

_{ red}
= _{ sp} / c

**relative viscosity [viscosity ratio]**,
_{ r}
Ratio of the viscosity in a suspension or solution to the viscosity of the suspending medium or solvent.

_{ r} = / _{ s}

**specific viscosity**,
_{ sp}
The
*relative viscosity* minus unity.

_{ sp} = _{ r} - 1

**zero shear viscosity**,
_{ 0}
The low
*shear rate* limiting value of viscosity. Associated with the first Newtonian region in many
*pseudoplastic* fluids.