They are definitely not the same – and they are commonly confused terms, so please translate them differently from each other, and consistent with each use! For me it helps to remember that retrievability is a property of the card, retention is a property of the person.
Retrievability is one of the core values of FSRS, so it’s best to keep the translation as close as possible to the precise definition. But basically – it’s the chance that a piece of information can be “retrieved” from your memory. “Memorability” might be another safe translation.
The probability that the person can successfully recall a particular piece of information at a given moment. It depends on the time elapsed since the last review and the memory stability (S).
Retention is used with different adjectives in different places, but it’s generally focused on how many cards you will remember when they are due and how many you did remember when you studied them. This is a good article about some of the various meanings when it is used.
- Desired retention
- Minimum recommended retention
- True retention
- Historical retention
- Average predicted retention