And well before Mylar there was something called rainbow paper that one needed to iron onto the embroidery, from memory.
Mylar. My neice sent me some. I am going to sew something with it this week.
My guess would be mylar - as well.
Is it mylar? Some designs are made with open spaces, where you can stitch a piece of mylar inside and it gives a neat shimmer, kind of irredescent glow. They are fun!