Origin: Germanic Old English suffix
Brightness has 4 different meanings across 1 category:
the location of a visual perception along a continuum from black to white
"The brightness of the gray swatch was adjusted until it matched the pale blue on the chart."
intelligence as manifested in being quick and witty
"Her brightness was evident when she solved the complex puzzle before anyone else could even read the instructions."
the quality of being luminous; emitting or reflecting light
"its luminosity is measured relative to that of our sun"
The quality of being bright.
"The brightness of the midday sun made it difficult to read without sunglasses."
In plain English: Brightness is how much light something gives off or reflects so it looks clear and not dark.
"The brightness of the noon sun made me close my eyes immediately."
The word "brightness" comes from the Middle English term for brightness or splendor, which itself evolved from an Old English root meaning light and radiance. It traveled into modern usage through a series of related Germanic languages before becoming part of standard English vocabulary.