advance payment vs retainer

advance payment

noun
  • The part of a contractually due sum that is paid or received in advance for a good, service or social security payment. 

retainer

noun
  • A fee one pays to reserve the other's time for services. 

  • A device that holds teeth in position after orthodontic treatment. 

  • Any thing or person that retains. 

  • A dependent or follower of someone of rank. 

  • A retaining valve. 

  • A paid servant, especially one who has been employed for many years. 

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