Patient-friendly definition from the DentistsDirectory.org dental glossary.
A dental retainer is an appliance worn after orthodontic treatment to hold teeth in their new positions while the bone and gum tissue surrounding the tooth roots remodel and stabilise around them. When orthodontic treatment moves teeth through the bone, the bone on one side dissolves (resorption) and new bone fills in on the other side. This bone remodelling takes months to complete — and until it does, teeth have a strong tendency to drift back toward their original positions. Retainers counteract this tendency.
Removable retainers come in two main types. Clear plastic retainers (Essix retainers) are thin, transparent trays that fit over the teeth — similar in appearance to aligner trays. They are nearly invisible and aesthetically preferred by most patients. Hawley retainers are made from acrylic with a metal wire that runs across the front teeth; they are more durable and adjustable but more visible. Fixed retainers are thin wires bonded to the back surfaces of the front teeth — typically the lower front teeth — providing passive retention 24 hours a day without requiring patient compliance. Many orthodontists provide both a bonded lower retainer and a removable upper retainer.
The honest answer: indefinitely. Teeth move throughout life as a natural process, independent of whether orthodontic treatment was ever done. Patients who wear retainers consistently after orthodontic treatment maintain their results well; those who stop wearing them almost always experience some degree of relapse. The consensus from orthodontic research is that retainers should be worn every night long-term, if not for life. If a retainer feels tight when you put it in, your teeth have shifted — keep wearing it consistently and see your orthodontist if it no longer fits at all.