Pointillism is a painting technique/movement, where the only brushstrokes that are used are dots to form images and patterns. Originally oil paint was used because/as it was thick and wouldn’t run on the canvas.
Georges Seurat and Paul Signac developed the technique in 1886, branching from the ideas used in Impressionism. – http://www.thewestologist.com/arts/of-paint-and-pixels.
When this technique is used, to create a blending effect colours are placed side by side, giving a small illusion of mixing when viewed.
Pixel art is very much like a modern version of pointillism, where as rather than dots, pixels are small squares.