periodical vs variable

periodical

adj
  • Periodic. 

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

  • 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. 

variable

noun
  • A quantity that may assume any one of a set of values. 

  • A shifting wind, or one that varies in force. 

  • A variable star. 

  • A symbol representing a variable. 

  • Something that is variable. 

  • Those parts of the sea where a steady wind is not expected, especially the parts between the trade-wind belts. 

  • Something whose value may be dictated or discovered. 

  • A named memory location in which a program can store intermediate results and from which it can read them. 

adj
  • Able to vary or be varied. 

  • Having no fixed quantitative value. 

  • Likely to vary. 

  • Marked by diversity or difference. 

  • Tending to deviate from a normal or recognized type. 

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