Personally I'd call it a collar, but I've never sewed anything in my life.
According to the people at Fabrics-store.com, who have sewn, it's called a collar panel.
Image text:
Along the slanted edge, press 3/8 inch (1cm) of the seam allowance towards the wrong side of the collar panel.
Pin the pressed edge of the collar panel along the front and back bodice panel. Sandwich the bodice seam inbetween the collar panel. Make sure the notched back centre mark matches with the stitched collar panel seams. Stitch along the pinned seam.
I also just learned that sewed and sewn are interchangeable!
