Definition
Brightness is how an object appears to shine (emit light) to the human eye.
Since it’s a perception, it depends of the actual physical luminance of the object, but also non-linearity in human perception of color, and optical effects such as White’s effect.
The center of this image appears brighter than it really is.
Not to be confused with Lightness or Luminance.
Example
A “perfectly white” pixel is brighter than a “black” one.
The contrast of an image is a range of brightnesses.
Tone mapping affects brightness of an image
Sources
https://en.wikipedia.org/wiki/Projection_screen#Image_brightness_and_contrast