A pillow tab (or satin tab) is a small square of satin, nylon or other silky fabric sewn in the back of the panty (or slip) waistband. It is sewn over the place where the elastic ends join or meet, usually at the center back of the panty though sometimes they are found at a side seam. The pillow tab usually wraps around both sides (inside and outside), but sometimes is only on the inside of the waistband.
Not only does this tab strengthen the joining of the elastic, but it does protect delicate skin from the irritating edge of the elastic seam — like a little pillow, hence the luxurious name of ‘pillow tab’ or ‘satin tab’.
This little lingerie luxury was common place on high-quality vintage panties and slips. However, the existence of a pillow tab does not guarantee the garment is vintage. While most modern lingerie manufactures do not make pillow tabs, I have seen some modern made panties, especially handmade panties and handmade sissy panties for men, with this nice touch.