The outer rim of the upper part of the ear, which curls. When pierced, the piercing is often referred to as a "cartilage piercing", although the term is no longer specific enough since different ear piercings are gaining popularity which are located in abnormal parts of the ear (all of which are through cartilage... except in the lobe).
"I got my cartilage pierced yesterday."
"which piercing did you get?"
"... my cartilage?"
"Oh, you mean your helix."