nape (of the neck), back of the neck
Los ojos que tienes sobre tu nuca.
The eyes you have on the back of your head.
Y tu tienes una encantadora y delicada nuca.
And you have a lovely, delicate nape.
— Fresas salvajes (1957)
Cada vez que se ríe, le veo la nuca.
Every time he laughs, I see the back of his neck.