Celtic button knot is a stopper knot on a single cord that results in a spherical decorative knot with hair braid / basket weave pattern. It is similar to the diamond knot. The main difference is that the diamond knot is tied using two cords. The Celtic knot on the other hand can be tied using a single cord.
The primary use for this knot is as a stopper knot. It typically is used as a button, or as a knot securing the end of the cord from fraying. It is often used for finishing certain paracord projects.