DIADEM (Gr. diadēma). Properly an emblem of royalty, but in the OT the Hebrew word is generally rendered “mitre” and refers to the turban of the chief priest (Zech.3.5), a royal diadem (Isa.62.3), or a turban (Job.29.14). In the NT the word diadem does not occur in the KJV or NIV, but the Greek diadēma is used three times in Revelation (Job.12.3; Job.13.1; Job.19.12) as an emblem of absolute power and is to be distinguished from the crown (Gr. stephanos) that is used elsewhere in the NT. The stephanos was given to victorious athletes, to generals and to the early emperors of Rome, until Diocletian (c. a.d. 284-305) transformed the empire into an Oriental absolutism and adopted the diadem as a symbol of his autocracy. Our Lord too will wear the diadem (Rev.19.12). See also Crown.
Davies-Mitchell, Student’s Hebrew Lexicon (1960), 311, 545f.; Zondervan, Interlinear Greek-English NT (1965), 25.
International Standard Bible Encyclopedia (1915)
di’-a-dem: There are seven Bible references to the diadem, four in the Old Testament and three in the New Testament. The Hebrew words do not mark any clear distinctions.
(1) tsaniph, tsanoph, tsaniphah (all from tsanaph, primarily "to wrap," "dress," "roll") mean a headdress in the nature of a turban or piece of cloth wrapped or twisted about the head. The word is also rendered "hood," "mitre." Job 29:14: "My justice was as a robe and a diadem" (RVm, "turban"); Isa 62:3: "a royal diadem in the hand of thy God."
(2) tsephirah, means "a crown," "diadem," i.e. something round about the head; Isa 28:5 "a diadem of beauty, unto the residue of his people."
(3) mitsnepheth, means an official turban or tiara of priest or king, translated also "mitre." Eze 21:26: "Remove the mitre, and take off the crown."
(4) diadema, the Greek word in the New Testament for "diadem," means "something bound about the head." Found 3 t, all in Re 12:3: "a great red dragon .... and upon his heads seven diadems" (the King James Version "crowns"); Re 13:1: "a beast .... and on his horns ten diadems"; 19:11,12: "a white horse .... and upon his head are many diadems." See Crown.