In the end I found out that ucipital mapilary is the suprasternal notch (or in Latin lacuna suprasternalis), the visible, little indentation between clavicles (collarbones, the pair of bones joining the breastbone to the shoulder blades), which is regarded as the mark of true beauty.
If you are by any chance a medical student, doctor, anatomist or sort of forensic expert, don't tell me there is no such term in Gray's Anatomy, because it's actually a fictional term coined by the screenwriter Samson Raphaelson for the 1941 Hitchcock thriller Suspicion, in which an exchange between Johnnie Aysgarth (Cary Grant) and Lina McLaidlaw Aysgarth (Joan Fontaine) went:
Johnnie: Don't do that.Remember what Count Laszlo de Almásy (Ralph Fiennes) said to Katharine Clifton (Kristin Scott Thomas) in The English Patient?
Lina: Why not?
Johnnie: Because your ucipital mapilary is quite beautiful.
I claim this shoulder blade. No, wait. I want – turn over – I want this, this, this place. I love this place. What's it called? This is mine. I'm going to ask the King permission to call it the Almásy Bosporus.'This place' is indeed ucipital mapilary.