headrest vs pillion

headrest

noun
  • The part of a seat designed to support the sitter's head. 

pillion

noun
  • The cushion of a saddle. 

  • The person riding in the pillion. 

  • A similar second saddle on a motorcycle for a passenger. 

  • A pad behind the saddle of a horse for a second rider. 

verb
  • To ride on a pillion. 

  • To put a pillion on a horse. 

adv
  • Riding behind the driving rider, as when positioned on the rump of a mount. 

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