The word retainer has two main meanings:

1. A device you wear in your mouth to keep your teeth in position. After getting braces removed, you'll likely wear a retainer at night to prevent your teeth from shifting back to where they started. It's usually made of plastic and wire and is custom-fitted to your mouth.
2. A fee paid in advance to keep someone available for their services when you need them. A business might pay a lawyer a monthly retainer so the lawyer stays available to answer questions and handle problems as they come up. Think of it as reserving someone's time and expertise, like paying to hold a spot. Today, consultants, attorneys, and other professionals may work “on retainer,” meaning they're paid regularly to be available rather than billing only when called upon for specific tasks.