Ecumenism (/ɪˈkjuːməˌnɪzəm/ ih-KYOO-mə-niz-əm; alternatively spelled oecumenism) – also called interdenominationalism, transdenominationalism, or ecumenicalism – is the concept and principle that Christians who belong to different Christian denominations should work together to develop closer relationships among their churches and promote Christian unity. The adjective ecumenical is thus applied to any interdenominational initiative that encourages greater cooperation and union among Christian denominations and churches. (Wikipedia)

Ecumenism is, principally, dialogue between Christian groups aimed at promoting the restoration of unity among all Christians through understanding, through mutual respect and toleration, and through practical cooperation in areas of common concern, such as care for the poor, sick, and needy.

Orthodox Christians were engaged in the foundation of the ecumenical movement from its inception. The primary basis for the Orthodox role in the ecumenical movement was a statement issued by the Ecumenical Patriarchate of Constantinople in 1920 entitled "Unto All the Churches of Christ Wheresoever They Be". A number of Orthodox Churches were present at the initial founding conference of the World Council of Churches, and most have continued to participate in the life of the WCC, as well as in national and regional councils of churches.

Some Orthodox Christians have criticized participation in the ecumenical movement. They believe that ecumenical witness represents a concession to the "Branch Theory," which suggests that the various divisions in Christianity all represent branches of the same Church just as the branches of a tree are all integrally part of the same living tree. Under such a model, Orthodoxy would be defined not as exclusively the One, Holy, Catholic, and Apostolic Church, but rather as a relatively small segment of the Church: one denomination among many. (OrthodoxWiki)