Gradient
A continuous, stepless transition between two visual states — most often color, value, or scale.
In practice
A gradient blends one visual state into another with no visible steps: a sky fading from pale yellow to blue on a travel poster, or a button that darkens slightly toward its base to suggest a curve. Gradients can also run through size, as when a field of dots shrinks across a page. Long, subtle gradients can show banding, visible stripes, so designers often add a little noise to smooth them.
Try this
Fill a rectangle with a two-color gradient, then add a third color in the middle, and compare which transition looks smoother.