the.com/retainer

means A sum paid upfront to secure someone's services on standby, or the device you wear to keep your teeth from wandering back to their crooked ways.

from From French retenir, 'to hold back' or 'keep,' built from Latin re- (back) plus tenere (to hold) — the same root that gives us tenant, tenacious, and contain. A retainer, in any sense, is fundamentally something that holds: a fee that holds a professional in reserve, a servant historically retained in a household's service, or a plastic contraption holding your teeth in line.