Concelebration is the act of multiple clergy conducting divine services together. Rubrics dictate what changes are made in the services when a concelebration occurs, as distinct from when the services are performed by only one priest or also a deacon. Concelebration is often a sign of full fellowship and communion between jurisdictions which recognize each other as fully Orthodox.
In the cases of tension between hierarchs or jurisdictions, concelebration between their respective clergy may sometimes be forbidden. Such a prohibition is not a schism or the same as a break in communion, which marks a more significant rupture in relations. Jurisdictions forbidding concelebration still recognize each other as fully Orthodox, but the prohibition of concelebrations may herald an eventual break in communion and possibly a full schism.