Plain English Meaning
How thick or thin a liquid is, and how easily it flows. Honey has high viscosity; water has low viscosity.
Full Definition
A measure of a fluid's resistance to flow. A high-viscosity fluid flows slowly and feels thick; a low-viscosity fluid flows easily and feels thin. In aviation, viscosity is a critical property of engine oils, hydraulic fluids, and fuels, and it changes with temperature — most fluids become less viscous as they warm up and more viscous as they cool.
Context Anchor
Seen in aircraft maintenance when choosing or checking engine oil, lubricants, fuel, and fluids that must move through lines or small openings.
Derivation
From the Latin viscum, meaning mistletoe — the berries of which produce a sticky substance once used as bird-lime. The word came to anything thick and sticky, then narrowed to the technical sense of resistance to flow.