canonical form vs headword

canonical form

noun
  • Synonym of dictionary form 

  • A standard or normal presentation of a mathematical entity or a text string, etc. 

  • Any of a set of representations of the resonance structure of a molecule each of which contributes to the real structure; a contributing structure. 

headword

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

  • Any word which may be modified by an adjunct. 

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