Halftone is a printing technology and a shading technique inspired by it, where a single color of ink is used to suggest a range of tones by filling the space to be shaded with either regularly-spaced circles of varying size, or circles of fixed size and varying spacing.

https://en.wikipedia.org/wiki/Halftone

With black ink on a white background, for example, larger or denser circles suggest darker shades of grey. The technique is also used in full-color CMYK printing and as a special effect in digital art.

See also

• dithering -- related technique on the scale of individual pixels
• screentone -- related technique that uses circles (or other shapes) but with fixed size and spacing