Nieashi (沸足) is a term used in the context of Japanese swords. It refers to a pattern where the ""Ashi"" - the line or dot that appears from the blade edge towards the tip - is composed of ""Nie"" (沸), the visible particles of hardened steel.

The term ""Nieashi"" translates to ""boiling foot"" in English. It is named so because the pattern appears as a long, rod-like shape extending from the blade edge (Habuchi) towards the blade tip, and it is composed of ""Nie"".

Depending on the shape of the ""Nieashi"", it can be further classified into different types such as ""Koashi"" (small foot), ""Sakaashi"" (reverse foot), and ""Nezumiashi"" (mouse foot).

Therefore, ""Nieashi"" adds a unique and interesting visual effect to the blade, enhancing its overall aesthetics and value.