term:

Instrumentality

instrumentality n

pl: -ties
1 : something through which an end is achieved or occurs [damages incurred in a single incident through an owned by the employer]
2 : something that serves as an intermediary or agent through which one or more functions of a larger controlling entity are carried out
: a part or branch esp. of a governing body compare alter ego

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