|Simultaneous contrast is what happens when the surroundings of an object affects the object itself.|
It refers to the visual influence of a color in close proximity to another when the two are viewed at the same time. The color in the greater quantity influences the other to chance in some way. This may be a hue, value, or chroma change or a combination of characteristics.
As your eyes keep scanning the object and its surrounding, trying to get an appropriate contrast for better depth-perception, they tend to neglect the object itself, concentrating only on its contrast with the surrounding.
Simultaneous contrasting can also be applicable to spacing and orientation as well, where the surrounding's spacing can influence the spacing of the object of interest.