third party 
noun
A person or group besides the two primarily involved in a situation, especially a dispute: the involvement of a third party as an independent valuer a sympathetic third party is of help when marriage conflict gets out of hand a third party in their conversation...
- In these circumstances, the asset given as security belongs to the defendant, not a third party.
- What I wanted to ask you was, was there any possibility of a third party being involved in the dismemberment?
- Here there is no need for a third party to have become involved.
adjective [attributive]
1Relating to a person or group besides the two primarily involved in a situation: third-party suppliers...
- Participating suppliers must submit to third-party audits and disclose energy sources to consumers.
- Many e-commerce websites use online shopping carts provided by third-party suppliers.
- There is no disinterested third-party witness to say who is telling the truth and who is lying.
1.1British (Of insurance) covering damage or injury suffered by a person other than the insured: third-party liability...
- By law, motorists must have at least third-party insurance cover just in case they cause an accident to another person or to another person's property.
- Note that a basic third-party insurance policy doesn't cover you for damage caused to yourself or your own car.
- And as her 22-year-old son has only got third-party insurance he will have to settle the bill personally.