Contrast
Juxtaposition of strongly opposing visual properties that sharpens the perceived identity of both.
In practice
Contrast places strongly opposed qualities side by side so each becomes clearer: large against small, dark against light, rough against smooth, heavy type against thin. It creates focus and makes things readable. On a website, black text on white has high tonal contrast, while one red button among gray elements uses color contrast to stand out. Too little contrast makes text hard to read, especially for people with low vision.
Often confused with Emphasis
Contrast is the difference between properties; emphasis is the result of using contrast and other means to make one element the dominant point of attention.
Try this
Set one word in five pairs of opposites, such as big and small or bold and light, and rank which pair grabs you most.