In other words, what do you call these?
(Each individual side?)
Most generically, they would be called lapels (the left lapel and the right lapel). And, to the best of my knowledge, it's absolutely irrelevant whether it's a coat or a bathrobe that you're talking about—they're still called lapels. There are different types of lapels such as notch lapels, peak lapels and so on and so forth, but they're all lapels.
That entire feature inside your circle is sometimes called a shawl collar. If you were shopping for such a garment, you would tell the salesperson that you wanted a bathrobe "with a shawl collar".
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!