A negative prompt is a separate text field where you list things the model should NOT include in the image. Useful for avoiding watermarks, extra fingers, blurry artifacts and unwanted style elements. Negative prompts have a big effect on Stable Diffusion and Flux 2 but a small or zero effect on GPT Image-2 and Imagen 4, which are trained on different objectives. Midjourney uses the --no parameter to express the same idea.