noun
Medical/anatomical term; in everyday language, 腎臓 (じんぞう) is more common.
See also: 腎臓
腎機能の低下が心配です。
I'm worried about the decline in kidney function.
腎移植のドナーを探しています。
We are looking for a kidney transplant donor.
腎臓 is the common word for 'kidney' in everyday speech. 腎 is more technical and often appears in compounds.
From Middle Chinese 腎 (dzyinX). The kanji is used in medical terminology for the kidney.