Feedback describes a cause-result relationship. Feedback is often thought of as a loop, with the cause and result feeding back information to each other.
In interaction-design the term feedback often refers to the loop of communication between the system or interface on the one side, and the user on the other side. Based on input or actions taken by the user, the system communicates status and result of said actions.
Feedback also refers to an opinion collected from users of a system/software/interface/process. The evaluation of user testing is often based on such feedback gained from users, ranging from short ratings to extensive questionaires.