Dantian丹田
dāntián
The "elixir field" below the navel where a cultivator stores and refines qi; the body's power core.
Chinese internal arts describe three dantian (lower, middle, upper), but fiction focuses on the lower one, a few finger-widths below the navel. It functions as a tank and furnace in one: absorbed qi is compressed there, refined to higher purity, and eventually crystallized into structures like the golden core.
The dantian is also the genre's favorite site of catastrophe. A crippled or shattered dantian means the end of a cultivator's path, which is why so many web novel protagonists start their stories as "trash" with damaged foundations, and why the eventual repair or replacement (a new technique, a mysterious golden finger) launches the comeback the whole story is built on.