headword vs locution

headword

noun
  • Any word which may be modified by an adjunct. 

  • A word used as the title of a section, particularly in a dictionary, encyclopedia, or thesaurus. 

locution

noun
  • The use of a word or phrase in an unusual or specialized way. 

  • A phrase or expression connected to an individual or a group of individuals through repeated usage. 

  • A supernatural revelation where a religious figure, statue or icon speaks, usually to a saint. 

  • Style of discourse or usage, or any particular utterance in such style. 

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