TT #290 · Parshas Tazria · Aliyah 7 · Passuk 55
English: The priest shall examine the lesion after it has been washed; if the lesion has not changed in appearance and has not spread, it is impure; you shall burn it in fire. It is a penetrating decay, whether the worn or unworn side is affected.
Torah Temimah cites Sifra (Toras Kohanim) for the meaning of the term pachatet as a sunken appearance.
Sifra — Vayikra 13, Midrash Sefer
English: The term “pachatet” means that all of the lesion's appearances are sunken.
Textual derivation
The description “pachatet” refers to an appearance that is sunken throughout.