One definition of a doodle is a drawing that you don't agonise over. You just do it. You don't think much about it. It may be something you do while you're on the phone or thinking something over. There's no judgement of how the piece looks, you don't organise or analyse anything. It just evolves.
Doodles/doodling have become an accepted art form in themselves. You can even get books on it.
Here are some kids from one of my notebooks. Mostly I do 'em in pencil and refine them with a fine black marker. Adding details makes them unique. Stay tuned for a lowdown on making some yourself.