Inazuma (稲妻) is a term used in the context of Japanese swords. It refers to a pattern that appears in the tempered edge of the blade. This pattern follows the grain of the blade's forging, and it appears as a thin, black, shining line that seems to cut through the ""Nie"" (沸) or the ""Nioi"" (匂). The term ""Inazuma"" translates to ""lightning"" in English, and the pattern is named so because it resembles lightning streaking through clouds, where the clouds are metaphorically represented by the ""Nie"" or the ""Nioi"".

In more detail, ""Nie"" refers to the visible particles of hardened steel that appear along the cutting edge of the blade, while ""Nioi"" refers to the whitish misty appearance also along the cutting edge, which is a more subtle hardening compared to ""Nie"". Both ""Nie"" and ""Nioi"" are results of the differential hardening process in sword making, and they contribute to the unique aesthetics of Japanese swords.

Therefore, an ""Inazuma"" pattern adds a dynamic and dramatic visual effect to the blade, enhancing its overall beauty and value."