Plain English Meaning
How thick or thin the oil is, and how easily it flows. Thicker oil flows slowly; thinner oil flows quickly.
Full Definition
A measure of an engine oil's resistance to flow. Higher viscosity oil flows more slowly and forms a thicker film between moving parts; lower viscosity oil flows more easily and circulates faster, especially when cold. Aircraft engine oils are graded by viscosity (for example SAE 50, SAE 60, or multi-grade designations like 15W-50), and the correct grade is selected based on engine type and outside air temperature.
Context Anchor
Seen in engine oil discussions, aircraft operating instructions, oil selection, and cold-weather engine operation.
Derivation
From the Latin viscum, meaning mistletoe, whose berries produced a sticky substance used to trap birds. The word came to describe anything thick and sticky, and then the technical property of resistance to flow.