There are multiple definitions - click on the translation that matches your context.

aktør (i sak)

substantiv

full

party

noun

Definitions of party

a person involved in a suit, dispute resolution proceedings or (less commonly) a prosecution

This version of events is clearly designed to cast Mr Mendelsohn as the innocent party.

a person taking part in a business transaction, negotiating a contract, bound by a contract etc

It was in the best interests of both parties to terminate the contract.