History of Christianity
Ecumenism broadly refers to movements between Christian groups to establish a degree of unity through dialogue.  "Ecumenism" is derived from Greek  (oikoumene), which means "the inhabited world", but more figuratively something like "universal oneness".  The movement can be distinguished into Catholic and Protestant movements, with the latter characterized by a redefined ecclesiology of "denominationalism" (which the Catholic Church, among others, rejects).