add-on vs top-up

add-on

noun
  • Something which can be appended to something else. 

  • A software extension or hardware peripheral that provides additional functions or customization for a core application or system. 

  • Additional chips that can be purchased at specific times in tournament play. 

top-up

noun
  • An addition. 

  • The situation where a student who holds a qualification equivalent to part of a degree course is then accepted onto a degree course at an intermediate point, without having to start it from the beginning. 

  • Additional credit purchased for a mobile phone. 

  • An additional premium paid over the initial premium in order to increase benefit values. 

  • A serving of drink used to top up an existing glass. 

  • A dose of epidural anesthetic added to previously injected spinal anesthetic in combined spinal-epidural anesthesia 

adj
  • That serves as an addition 

How often have the words add-on and top-up occurred in a corpus of books? (source: Google Ngram Viewer )