Yō (葉) is a term used in the context of Japanese swords. It refers to a pattern that appears on the blade during the tempering process. In this pattern, the ""Nie"" (沸) - the visible particles of hardened steel - or ""Nioi"" (匂) - the whitish misty appearance along the cutting edge - detach from the head or valley of the tempering and appear independently within the blade, scattered in small elliptical shapes.
The term ""Yō"" translates to ""leaf"" in English. It is named so because the pattern resembles leaves fluttering down. This feature is often seen in the works of swordsmiths from the Bizen tradition, one of the five main traditions of Japanese sword making.
Therefore, ""Yō"" adds a unique and beautiful visual effect to the blade, enhancing its overall aesthetics and value.