Retainer

noun
1.
a person or thing that retains.
2.
a servant or attendant who has served a family for many years.
3.
Also called cage, separator. Machinery. a ring separating, and moving with, balls or rollers in a bearing.
4.
Orthodontics.

a fixed or removable device worn in the mouth to hold the teeth in their new position during the adaptive period after straightening appliances have been removed.
Prosthodontics. a part on a bridge or the like by which the bridge is attached to the natural teeth.

noun
1.
the act of retaining in one’s service.
2.
the fact of being so retained.
3.
a fee paid to secure services, as of a lawyer.
noun
1.
(history) a supporter or dependant of a person of rank, esp a soldier
2.
a servant, esp one who has been with a family for a long time
3.
a clip, frame, or similar device that prevents a part of a machine, engine, etc, from moving
4.
a dental appliance for holding a loose tooth or prosthetic device in position
5.
a fee paid in advance to secure first option on the services of a barrister, jockey, etc
6.
a reduced rent paid for a flat, room, etc, during absence to reserve it for future use

retainer re·tain·er (rĭ-tā’nər)
n.

One that retains, as a device, frame, or groove that restrains or guides, especially for a prosthesis.

An appliance used to hold teeth in position after orthodontic treatment.