A doodle is a casual little drawing you do, often while you're doing something else, like talking on the phone or listening to a teacher's lecture. Your history notebook might be equal parts notes and doodles.
A doodle can vary from a meaningless scribble or scrawl to a detailed picture of a dog. You never draw a doodle for some specific purpose — to doodle is to draw in an aimless, haphazard way. In the 1600s, a doodle was a "simple fellow," from the German dudeltopf, "simpleton." It came to mean "foolish designs on paper" by the early 20th century.