**Contrast** is a difference between any two things or sensory inputs. 

Hues can contrast (see [[wp>complementary colors|complementary colors]]). 
Values can contrast. Shapes can contrast (e.g. round shapes contrast pointy shapes).
Lines can contrast (horizontals contrast verticals, wavy lines contrast straights).

Pretty much any thing has the potential to create a strong perceptual difference between itself and something else.