term:

Intervene

intervene vi

-ven·ed
-ven·ing
1 : to occur, fall, or come between points of time or events [may be held liable even though other independent agencies between his negligence and the ultimate result "Hooks SuperxInc. v. McLaughlin, 642 N.E.2d 514 (1994)"]
2 a : to come in or between by way of hindrance or modification [ to stop a fight]
b : to become a party to a legal proceeding begun by others in order to protect an alleged interest in the subject matter of the proceeding [the Commissioner of Internal Revenue was granted leave to "P. A. Freund"] compare implead, interplead, join
in·ter·ve·nor [-vē-nər, -nȯr]
n

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.