Pixel
One tiny square of colour; a picture on a screen is a grid of them.
See it in Pixel art editor. Turn on Show grid, then draw: every square you fill is one pixel.
In plain words
A screen is a grid of tiny squares, and each one shows a single colour. That square is a pixel. From a normal distance the squares blur together into smooth pictures. Each pixel stores its colour as numbers for red, green and blue, and sometimes for alpha. A photo from a phone holds about twelve million of them. Pixel art keeps them large on purpose, so every square is a choice.