What is Omaru boshi in Japanese Katana terminology?

The term "大丸帽子" (Oomaru boshi) refers to a type of tempering pattern on a Japanese sword where the tempered edge part is large and returns in a large arc along the curvature of the sword. The term "Oomaru boushi" is used to describe this pattern where the tempering width around the curvature is shallow, resulting in a large return angle of the tempering pattern. This pattern is often seen in older swords and is characterized by its strong impression due to the thin return compared to the tempered blade part.