What is the name called in the pyramid in your back?

This is an amazing question. I'm just going to spout answers to what I think you might be asking, and we'll see if anything sticks.

The triceps brachii is a "three-headed" muscle in the back of your arm, used to straighten it.

The triangular bone behind your shoulder is the scapula, which connects your upper arm to your collar bone.

On the back of the Great Seal of the United States (and subsequently on the back of the dollar bill) is a 13-level unfinished pyramid, with the Eye of Providence where the top would be.

If you are on the bottom of a human pyramid, with people kneeling on your back, you form part of the base.