mediator

Concise Oxford English Dictionary © 2008 Oxford University Press:
mediate

▶verb /ˈmiːdɪeɪt/ 

• 1 intervene in a dispute to bring about an agreement or reconciliation.

• 2 technical be a medium for (a process or effect).

▶adjective /ˈmiːdɪət/ involving an intermediate agency.

– derivatives
mediately adverb,
mediation noun,
mediator noun,
mediatory /ˈmiːdɪət(ə)ri/ adjective.

– origin ME (as an adjective in the sense ‘interposed’): from late L. mediatus, mediare ‘place in the middle’, from L. medius ‘middle’.