(covers information from several alternate timelines)
A liaison was an individual who acted as an intermediary between two parties. This person could be officially designated as a liaison officer.
In 2267, Governor Kor appointed "Baroner" (who had been described as a leading citizen of the Organians) as a liaison between the occupying forces (the Klingons) and the civil population (the Organians). (TOS: "Errand of Mercy")