The Ecumene is the formal name of the galaxy-spanning Forerunner empire which existed over 100 thousand years ago and consisted of three million or more planets. The political center of this empire was known as the Capital, a massive structure containing multiple cities.
"Ecumene" is a word describing the earth as known by the classical civilizations of Greece and the Romans. This later became used by Christian denominations to refer to the general "Christian world".
Even though the Ecumene council could number in the hundreds, there seems to be three main speakers; the Didact, the Librarian and Faber, who represent the Warrior-Servant caste, the Lifeworker caste and the Builder caste respectively.