IPA: /rɪˈteɪnər/
KK: /rɪˈteɪnər/
A person who serves or works for someone, especially in a household, or a device used in dentistry to keep teeth in place after treatment.
After finishing her braces, she wore a retainer to keep her teeth straight.
Retainer → It is formed from "re-" (meaning back or again) and "tenere" (meaning to hold). The word refers to something that holds or keeps something back, often used in the context of a person who is retained for service.
Think of something that 'holds back' ('tenere') again ('re-') — that's why a retainer is something that keeps or holds something in place.