A diadem is a crown worn by a king or royalty. It can also signify holiness.
Diadem only occurs a couple of times in the Bible, all in the Old Testament. Usually it’s in reference to the first priest, Aaron, on whose head it was place by Moses. Interestingly, the diadem was attached to his turban. In this instance, it is called a sacred diadem.
Isaiah refers to a royal diadem or crown of splendor as a visual image of how much God values his people.