From Longman Dictionary of Contemporary Englishthird partyˌthird ˈparty1 noun [countable] lawSCL someone who is not one of the two main people involved in an agreement or legal case, but who is affected by it or involved in it in some way
Examples from the Corpus
third party• Is it lawful to refer questions of fact and law to a third party to determine in a non-judicial fashion?• Normally, that step is the reference of a dispute to an independent third party.• Both companies will meet with a neutral third party to resolve the disagreement.• It also produces spirits locally, either directly or through third party arrangements.third partyˌthird ˈparty2 adjective → third party insurance/cover/policy → comprehensiveFrom Longman Business Dictionarythird partyˌthird ˈparty noun (plural third parties) [countable]1LAW someone who is not one of the two main people or organizations involved in an agreement or legal caseThe preference tests were conducted by an independent third party.2INSURANCE in an insurance contract, someone who is not the insurer or the insured person, but who will get money if the insured person causes them loss or injuryThe insurance only covers loss or injury to a third party. 