intermediary

Short definition from Wiktionary

Adjective

1. 1
   That intermediates.

Noun

1. 1
   An agent acting as a mediator between sides to facilitate agreement or cooperation.
2. 2
   A person or organisation in an intermediate position (in a transaction, agreement, supply chain, etc.)
3. 3
   One or several stages of an event which occurs after the start and before the end.