nocturnal vs per diem

nocturnal

adj
  • Taking place at night, nightly. 

  • Primarily active during the night. 

noun
  • A device for telling the time at night, rather like a sundial but read according to the stars. 

  • A person or creature that is active at night. 

per diem

noun
  • A daily stipend. 

  • A daily allowable limit for expendables. 

adj
  • per day; daily. 

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