An agreement was a manifestation of mutual assent by two or more persons to one another.
The interjection "okey-dokey", frequently used by Ensign Sam Rutherford whenever he went agreed to comply with another person's request. (LD: "Envoys", et al.)
In 2367, while having a talk with Endar about Jeremiah Rossa, Jean-Luc Picard said to him, "You should have notified the Federation authorities. To conceal him was a clear violation of our agreement." (TNG: "Suddenly Human")