If you and your friends launch into singing “Old MacDonald” loudly together, in the middle of the public library, you will all have made a concerted, or cooperative, effort to annoy lots of people.
Concerted is often used to describe a group effort to achieve a particular goal. As you may have guessed, concerted comes from the word concert. While a concert can be defined as "a group of musicians playing cooperatively," concert can also designate other types of cooperative behavior. If a group of people do anything together — from singing an annoying song to throwing library books at agreed-upon targets — they are acting in concert, or in a concerted manner.