noun

1. a person or thing that retains
2. a person serving another, esp. someone of rank; servant, attendant, adherent, etc.
3. any of several devices used to retain; specif.,
   1. a groove, frame, etc. within which roller bearings are held
   2. a device designed to hold teeth in position after they have been adjusted by orthodontics

noun

1. the act of engaging the services of a lawyer, consultant, etc.
2. a fee paid in advance to make such services available when needed

Etymology: ME reteyner: see retain & -er, sense