A fluid‘s internal resistance to flow — the measure of how ‘thick‘ it is. Viscosity governs honey‘s slow pour, ketchup‘s reluctance to leave the bottle, and oil‘s lubricating glide. In food processing, viscosity determines pumping energy, heat-transfer rates, mixing efficiency, and filling accuracy; in the mouth, it shapes perceived body and creaminess. Temperature dependence is steep — most foods thin dramatically when heated — so processes must be designed around viscosity at operating conditions. Expressed in pascal-seconds (Pa·s), viscosity is the single most practical rheological parameter linking formulation, processing, and sensory quality.