Congratulations, you're human!
The feature is called the "inion" and is part of the region of attachment for what's called the "nuchal muscles". The shape of the inion is
slightly different in men and women, and is one of the methods of determining a male skull from a female skull.
en.wikipedia.org...:Gray1193.svg
Or if you meant the one further down, it's the external occipital protrubance
en.wikipedia.org...:Gray129.png
W have lots of lumps and bumps and interesting things on our skulls and bodies, but we're not used to examining them in close detail as a rule.