I've encountered the phrase small of one's back often when I was reading the Divergent series, and recently encountered it again on a Wikipedia article. I've searched its meaning on the internet, but I'm still confused as to where exactly it is located in the body. Some says it's near the buttocks , some says it's higher up, on the part where the usual "curve" is on the torso area. Where did the term came from? Where is it exactly? Is there a single word for it?