retainer

n. person or thing which retains; servant, household employee; initial fee paid in order to obtain services from a professional (such as a lawyer); dental device used to hold teeth in place after straightening (Dentistry)

A retainer can be: a vassal in feudal Japan, often Samurai providing military servicesa person being part of the retinue (suite, train) of a dignitary etcetera, either performing personal services such as man servant, pageboy, bodyguard, or rather occasionally in proximity such as a minstrelthe part of a contractually due sum that is paid in advance, while the balance will only follow after receipt on the counterpart in goods or services; also called advance payment or deposit a device in dentistry: Retainer (orthodontic device) used to correct or keep teeth correctedan individual, in civil proceedings, assigned to maintain assets of an organization; for example, in a lawsuit between shareholders, a judge may be asked to replace an executive who holds shares with a retainer if they believe there is a danger of the executive running the company into the groundRFA Retainer (A329), an armament support shipa plastic substitute post-earring to prevent the closing of a piercing
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Retainer /re·tain·er/ an appliance or device that keeps a tooth or partial denture in proper position. re·tain·er n. 1. [more]Retainer - Community and Resources

Noun
1. a fee charged in advance to retain the services of someone
(synonym) consideration
(hypernym) fee
(hyponym) quid pro quo, quid
2. a person working in the service of another (especially in the household)
(synonym) servant
(hypernym) worker
(hyponym) body servant
3. a dental appliance that holds teeth (or a prosthesis) in position after orthodontic treatment
(hypernym) dental appliance

(n.)
The act of withholding what one has in his hands by virtue of some right.
  

(n.)
The act of keeping dependents, or the state of being in dependence.
  

(n.)
The act of a client by which he engages a lawyer or counselor to manage his cause.
  

(n.)
One who, or that which, retains.
  

(n.)
One who is retained or kept in service; an attendant; an adherent; a hanger-on.
  

(n.)
Hence, a servant, not a domestic, but occasionally attending and wearing his master's livery.
  

(n.)
A fee paid to engage a lawyer or counselor to maintain a cause, or to prevent his being employed by the opposing party in the case; -- called also retaining fee.