diadem

Diadem comes from Middle English diademe, from Old French diademe, from Latin diadēma, from Ancient Greek διάδημα (diádēma), meaning a band worn around a tiara. That Greek noun comes from the verb διαδέω (diadéō), "to bind around."

The word began as a plain description of a band tied around the head, and stayed close to that sense as it passed through Latin, French, and Middle English into modern usage.

It shares its root with the Greek verb diadéō, "to bind around" — the diadem is literally the thing that binds.