The noun nonentity refers to a person of no significance or importance. If you are a member of a sports team, but spend all your time sitting on the bench watching the rest of the team play, you will probably feel like a nonentity.
Something that is an entity exists, and if you are considered a nonentity, you might as well not exist. A political nonentity has no power, and a social nonentity blends in with the wallpaper. Not only people are considered nonentities; groups or things can be nonentities too. If a nonentity beats a nationally ranked team, that's a real upset. The cereal brand that stays on the shelf and is ignored can also be considered a nonentity in the cereal market.