The kanji 髄 primarily signifies "marrow," specifically the soft, fatty tissue inside bones. It can also extend to represent the essence or core of something, the fundamental or most important part.

・On-yomi (音読み): ズイ (zui) ・Kun-yomi (訓読み): None

⚫︎ 骨髄 (kotsu zui) Translation: Bone marrow ⚫︎ 脊髄 (sekizui) Translation: Spinal cord ⚫︎ 髄液 (zuieki) Translation: Cerebrospinal fluid

The kanji 髄 is a phono-semantic compound. The left side is 月 (tsuki) which means "moon" or, in this context, it is a simplified version of 肉 (niku), which means "meat" or "flesh", indicating something related to the body. The right side is 遂 (sui), which provides the sound and conveys the meaning of "complete" or "to accomplish". So, 髄was created to depict the essence or the "core" of the body (bone marrow), highlighting its important function in the body.