interposer

/ˌɪntərˈpoʊzər/
noun
  1. A person or thing that comes between others, often to mediate, interfere, or act as a go-between.
    • The teacher acted as an interposer when the two students started arguing.
    • In the negotiation, a neutral interposer helped both sides reach an agreement.
    • The software acts as an interposer between the user and the database.