Definitions:

1. fit in or on an existing structure, such as an older house verb
   • The mansion was retrofitted with modern plumbing
2. provide with parts, devices, or equipment not available or in use at the time of the original manufacture verb
   • They car companies retrofitted all the old models with new carburetors
3. substitute new or modernized parts or equipment for older ones verb
   • The laboratory retrofitted to meet the safety codes
4. the act of adding a component or accessory to something that did not have it when it was manufactured noun
   • the court ordered a retrofit on all automobiles