匂口 (Nioiguchi)

What is Nioiguchi in Japanese Katana terminology? 

Nioiguchi refers to the boundary between the blade pattern (Hamon) (Hammon) and the ground iron (Jigane) in a Japanese sword. Depending on the condition of the Japanese sword, it can appear tightened, moistened, or disappear. It is said to be an important part that represents the quality of a Japanese sword.

In other words, Nioiguchi is the line that separates the tempered area (the blade pattern (Hamon)) from the rest of the blade (the ground iron). The appearance of this line can change based on the condition of the sword, and it can appear more defined (tightened), more diffused (moistened), or it can even disappear entirely. The state of the Nioiguchi is considered a crucial factor in evaluating the quality of a Japanese sword.