I saw this effect of concentric circles discretely fading out (background of the image one).
I thought this was a circular gradient (radial-gradient).
But other images didn't seem to have a similar effect when I searched the term.
And another similar image that I saw uses the same kind of effect but it is not circular.
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Then I thought the effect I'm looking for was discrete color gradient. Which lead me to this page.
Is there a standard name for this kind of effect?