What is Indispensable Party?

Legal Definition
An indispensable party (also called a required party, necessary party, or necessary and indispensable party) is a party in a lawsuit whose participation is required for jurisdiction or the purpose of rendering a judgment. In reality, a party may be "necessary" but not indispensable. For example, if s/he claims an interest in the litigation, that interest may be impeded if s/he is not joined. That doesn't transform him or her into an indispensable party unless her absence threatens some other party's interest. Often, an indispensable party is any party whose rights are directly affected by disposition of the case. Many jurisdictions have rules which provide for an indispensable party to be joined (brought into the case as a party) at the discretion of the judge. In some cases, the inability to join such a party means that the case must be dismissed. This is outlined in the Federal Rules of Civil Procedure, Rule 19.
-- Wikipedia
Legal Definition
the term that is given to a party in a law suit who is essential in determining the suit.