diadem vs imperium

diadem

noun
  • Regal power; sovereignty; empire—considered as symbolized by the crown. 

  • A crown. 

  • An ornamental headband worn as a badge of royalty. 

  • An arch rising from the rim of a crown (rarely also of a coronet), and uniting with others over its centre. 

verb
  • To adorn with a diadem; to crown. 

imperium

noun
  • Supreme power; dominion. 

  • The right to command the force of the state; sovereignty. 

How often have the words diadem and imperium occurred in a corpus of books? (source: Google Ngram Viewer )