The ridge of the ear that runs from the back edge around, over the top, to inside the outer ear, where it becomes the daith. The traditional "cartilage" ear piercing is actually just below the helix, where the cartilage thins out a bit. A true helix piercing goes out radially in the plane of most of the ear, as opposed to the more traditional percing in that area which is roughly perpendicular to the upper conch.

If a left ear looks something like this, the helix is the part linked or bold:

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