rugby vs rugby union

rugby

noun
  • A form of football in which players can hold or kick an ovoid ball; rugby football. The ball cannot be handled forwards and points are scored by touching the ball to the ground in the area past the opponent's territory or by kicking the ball between goalposts and over a crossbar. 

  • The form of the game known as rugby union (see the usage note). 

  • Rubber cement, contact cement. 

verb
  • To play rugby. 

rugby union

noun
  • A form of rugby football, historically amateur, having 15 players per side; points are scored for a try, penalty, drop kick or conversion. 

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