1) One with backing 2) Party 
• (n.) The person who is vouched, or called into court to support or make good his warranty of title in the process of common recovery.
Vouch·ee' noun (Law) The person who is vouched, or called into court to support or make good his warranty of title in the process of common recovery. Blackstone. 
[n] - (law) a person called into court to defend a title
noun (law) a person called into court to defend a title