Plain English Meaning
How bright something looks to your eyes. A snow-covered field at noon has high luminance; a dark taxiway at night has low luminance.
Full Definition
The measurable amount of visible light coming from a surface in a given direction, as perceived by the human eye. It describes how bright a surface appears, taking into account both the light it emits or reflects and the area over which that light is spread.
Context Anchor
Seen in night flying and night vision discussions, especially when explaining why objects can be hard to see or can appear misleading in low light.
Derivation
From the Latin lumen, meaning light. The -ance ending turns it into a measurable quality — literally, the quality of giving light.