What is the art term for a preliminary sketch or plan?

In art, a thumbnail is a small, quick, rough sketch used as a preliminary plan for a composition. Thumbnail sketches are typically done rapidly at a reduced scale so the artist can explore different arrangements, viewpoints, and layouts before committing to a full-size work. They help the artist visualise the overall composition without investing significant time or detail.

A prototype is a preliminary model of a product or design, used more in industrial and product design than in fine art. Composition refers to the arrangement of elements within an artwork, not to a preliminary sketch itself. A draft is a preliminary version of a written document; while sometimes used loosely in art, it is not the standard academic term for a preliminary sketch.