A device used to hold teeth in position after orthodontic treatment, or a fee paid in advance to secure professional services.
From Old French retenir 'to hold back,' from Latin retinere, from re- 'back' + tenere 'to hold.' The legal sense of advance payment emerged in the 16th century, while the dental appliance meaning developed in the 20th century.
The dual meaning of 'retainer' beautifully illustrates how language evolves - both definitions involve 'holding something in place,' whether it's teeth in your mouth or a lawyer's commitment to your case. Medieval retainers were also servants 'retained' in a lord's household!
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