Yours is a definition of contrast, but not the one used in design or vision science [1, 2], where contrast is defined as a difference in luminance or color between regions of an image that makes objects and textures visible. If an image has no contrast, it is a solid patch. If an image has low contrast, the brightest or most colorful regions are not too different from the other regions, as if seen through a dense fog. If an image has high contrast, the brightest or most colorful regions are quite different from the other regions.

1. https://en.wikipedia.org/wiki/Contrast_(vision)

2. https://www.smashingmagazine.com/2014/09/design-principles-c...

Here we're talking specifically about "visual contrast", which has a meaning in perception slightly different than the common usage of "contrast".

Visual contrast is measured on a spectrum that starts at zero and goes up from there - a box of a particular color inside a box of the same color would have no contrast, but if there is any difference in the color values then there is at least some visual contrast.

Wikipedia isn't a great source for definitions, but has a reasonably thorough take on this https://en.wikipedia.org/wiki/Contrast_(vision), going into the details of contrast sensitivity, which deals with almost-imperceptible levels of contrast.