third party

noun

1. any party to an incident, case, quarrel, etc., who is incidentally involved.
2. (in a two-party system) a political party formed as a dissenting or independent group from members of one or both of the two prevailing major parties.

noun

1. a person who is involved by chance or only incidentally in a legal proceeding, agreement, or other transaction, esp one against whom a defendant claims indemnity

adjective

2. insurance providing protection against liability caused by accidental injury or death of other persons or damage to their propertythird-party insurance