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.
Hierarchical services are a special type of concelebration including one or more bishops, significantly altering the text and movement in a service.
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.