double date vs significant other

double date

noun

  • A (romantic) date that two couples go on together. 

verb

  • To go on a double date. 

significant other

noun

  • One's exclusive romantic partner. 

  • A person with whom one has an important bond of some kind. 

How often have the words double date and significant other occurred in a corpus of books? (source: Google Ngram Viewer )