The Catacomb Church (Russian: Катакомбная церковь) as a collective name labels those representatives of the Russian Orthodox clergy, laity, communities, monasteries, brotherhoods, etc., who for various reasons, have moved to an illegal position since the 1920s. In a narrow sense, the term "catacomb church" means not just illegal communities, but communities that rejected subordination to the Deputy Patriarchal Locum Tenens Metropolitan Sergius (Stragorodsky) after 1927, and adopted anti-Soviet positions. During the Cold War the ROCOR popularized the term in the latter sense, first within the Russian diaspora, and then in the USSR by sending illegal literature there. As a synonym for the "catacomb church" in this sense, the term True Orthodox Church (Russian: истинно-православная церковь, tr. istinno-pravoslavnaya tserkov) is also used, but as the historian Mikhail Shkarovsky notes: "the catacombness of the Church does not necessarily mean its intransigence. This term covers all unofficial and therefore not state-controlled church activities".
Organizationally, "catacomb" communities were usually not connected (organizations existed only in the NKVD cases). Accordingly, it is difficult to define the general ideology of the movement. In the underground were both communities that were quite loyal to the Moscow Patriarchate but did not have the opportunity to register and gather legally, and communities who believed that the power of the antichrist had come and that there could be no contact with the official church. Despite the absence of a common ideology and any organization, the Catacomb Church existed as a religious community and subculture.