Definition:
Couching is the term used to describe securing a length of thread or yarn on the surface of the canvas by tacking it in place with small stitches.
Thread and yarn couched to a surface of a design can be fine, thick, or metallic, floss or ribbon, and is meant to float on the surface of the design, rather than become part of the background area. It is secured by anchoring it down with small stitches.