The phrase "crease shade" refers to applying a darker color of eyeshadow in the crease of the eyelid to create dimension and depth in a makeup look. Full definition
I used to spend time selecting a palette and using multiple brushes to blend out my look — usually a matte crease shade, shimmery lid color, an inner corner and brow bone highlight, and darker shade on the outer third of my eye as liner — but now I use just this one product for equally glamorous results. (health.com)
Keep the eyes neutral with no more than a soft crease shade and winged liner.» (makeup.com)
It makes for a great crease shade, or even an all over bronzy / coppery lid color. (justalittleblush.com)