Boshi (帽子/鋩子) refers to the hamon pattern in the area of the tip of a tanto or sword. In most historical cases the hamon turns back for a distance so that the tip is hardened on both the spine and edge.
The most common pattern is called maru, which means rounded, and it is further categorized by the relative size of the radius as the hamon turns back, ko-maru is smaller, o-maru is larger.
Other notable patterns include tsukiage (coming to a sharp point), hakikake (like brush marks or a sand wash), and yakitsume (straight with no turnback). The distance of the turnback also varies depending on the smith and the type of tanto, and may even travel back most of the spine on certain types of yoroidoshi tanto.
See more photos of the tanto pictured above: islandblacksmith.ca/2015/05/yozakura-tanto/