Intermediary

adj. intermediate, located between; mediatory, acting between persons or parties

n. mediator, arbitrator, go-between, middle-man; means, intermediate agent; intermediate stage

For a publishing house, associated with the name of Leo Tolstoy see Intermediary (publisher) An intermediary is a third party that offers intermediation services between two trading parties. The intermediary acts as a conduit for goods or services offered by a supplier to a consumer. Typically the intermediary offer some added value to the transaction that may not be possible by direct trading.
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Noun
1. a negotiator who acts as a link between parties
(synonym) mediator, go-between, intermediator, intercessor
(hypernym) negotiator, negotiant, treater
(hyponym) mediatrix

(n.)
One who, or that which, is intermediate; an interagent; a go-between.
  

(a.)
Lying, coming, or done, between; intermediate; as, an intermediary project.
  

See: financial intermediary .