The property of a fluid, such as a printing ink, that describes the degree of its resistance to flow, or its ability to adhere to a surface. A fluid that is highly viscous, such as molasses, is sticky and glutinous, and does not flow easily. A fluid that is not very viscous, or "inviscid," such as water or alcohol, flows freely. In ink terminology, viscosity refers to the extent to which ink will resist flowing. The viscosity of an ink will depend on the printing process it is designed for, and the nature of the substrate to which it will adhere. An ink's viscosity is a component of its body. The viscosity of a particular ink can vary according to the stresses to which it is subjected. (See Thixotropy.)

Viscosity is measured in a metric unit called a poise, which is equal to the viscosity of a fluid in which one dyne per square centimeter is required to maintain a difference in velocity of one centimeter per second between two parallel planes in the fluid that lie in the direction of flow and are one centimeter apart.

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