noun
nape of the neck; nucha
Refers to the back of the neck, especially in contexts of beauty or vulnerability. The medical term 'nucha' is a direct equivalent.
彼女はうなじの産毛が美しい。
The fine hair on the nape of her neck is beautiful.
うなじを冷やすと気持ちがいい。
It feels good to cool the nape of your neck.
首筋 refers more broadly to the neck area, including the sides and front, while うなじ specifically means the back of the neck.
The origin is uncertain, but it may be related to 項 (うな, 'back of the neck') + じ (possibly a suffix). The kanji 項 is used for its meaning, but the word is now predominantly written in kana.