Communion of saints, is a belief which is professed in the Apostles’ Creed affirming the unity of the faithful, both the living and the dead, in union with Jesus Christ and with each other. It is the union of christian church members: alive, dead, in heaven in purgatory or in purification.
Saint Nicetas of Remesiana was the first to use this term.
The Latin communio sanctorum could also mean “communion of holy things,” especially sharing in the sacraments. A.G.H.
Bowker, John, The Oxford Dictionary of World Religions, New York, Oxford University Press, 1997, p. 229