Intervention
WordNet
noun
(1) (law) a proceeding that permits a person to enter into a lawsuit already in progress; admission of person not an original party to the suit so that person can protect some right or interest that is allegedly affected by the proceedings
"The purpose of intervention is to prevent unnecessary duplication of lawsuits"
(2) The act of intervening (as to mediate a dispute)
(3) A policy of intervening in the affairs of other countries