biweekly vs periodical

biweekly

adj
  • Occurring once every two weeks; fortnightly. 

  • Occurring twice a week. 

noun
  • Something that is published or released once every two weeks. 

adv
  • Every two weeks. 

  • Twice a week (but see the Usage notes). 

periodical

adj
  • Published at regular intervals of more than one day, especially weekly, monthly, or quarterly. 

  • Periodic. 

  • Of, or relating to such a publication. 

noun
  • A regularly issued thematic publication that contains the most current information in its field, often the primary means for communication of original scholarship or creative work at the cutting edge of research in its field. 

  • A publication issued regularly, but less frequently than daily. 

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