The invisible barrier between the story's world and the audience that keeps fiction feeling like a self-contained reality.
The fourth wall is the imaginary boundary between the characters in a story and the audience experiencing it. The term comes from theater, where a stage has three physical walls and the fourth is the open side facing the audience. In fiction, the fourth wall is the convention that characters do not know they are in a story and cannot see or address the reader. When this convention is maintained, the story feels like a window into a real, self-contained world.
Most stories you write will maintain the fourth wall, and understanding it helps you be intentional about it. The fourth wall is what allows your reader to feel immersed, to forget they are reading, to lose themselves in the experience. Knowing the wall exists also means you can choose when and how to break it for deliberate effect - but you need to understand the default before you can subvert it.
Jane famously addresses the reader directly ("Reader, I married him"), creating a thin fourth wall that makes the story feel like an intimate confession.
The frame narrative of a grandfather reading to his grandson constantly plays with the fourth wall, layering fiction within fiction.
The protagonist's glances and asides to the camera create a fourth-wall dynamic where the audience becomes her secret confidant.
Watch for moments where your narrator's voice slips and accidentally addresses the reader when the story has not established that convention. Consistency matters.
Fiction has a fourth wall too. Any time your prose calls attention to itself as prose, or a character seems aware of being in a book, you are interacting with the fourth wall.
Write a one-page scene in close third person that maintains a solid fourth wall. Then rewrite the same scene with the character suddenly aware they are in a story. Notice how the tone, tension, and reader experience change completely.