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1. Literally, performing a musical concert. They're one of my favorite bands, but strangely I've never seen them in concert.
2. Together; in cooperation. The two of us will need to work in concert if we're going to get through all these files. The local police department has been investigating in concert with federal agencies.
in concert (with someone)
Fig. in cooperation with someone; with the aid of someone. Mrs. Smith planned the party in concert with her sister. In concert they planned a lovely event.
Together, jointly, as in They worked in concert on the script, or When mind is in concert with body, one can accomplish a great deal. This expression uses concert in the sense of "an agreement of two or more persons." [Early 1700s]