to jointly recite the canon of the Eucharist in unison with others
• (v. t.) To celebrate together.
Con·cel'e·brate transitive verb [ Latin concelebratus , past participle of concelebrare to concelebrate.] To celebrate together. [ Obsolete] Holland. 
concelebrate 1. To celebrate together. 2. To take part in a concelebration of the Eucharist.