A person that believes the earth is flat. A flatearther believes NASA is a government agency that hides the truth of the flat earth with lies, fake photos, and videos of space. One believes the sun and moon are directly overhead with a diameter of roughly 30-70 miles across. Flathearther's typically suffer from the Dunning-Kruger effect.
Joe is now a flatearther after watching a bunch of youtube videos. He believes the world is flat.