noun Third Party
plural Third Parties
Third party refers to a person who is not part of the legal relationship that you are considering.
There are usually two parties to a contract, so the "third party" is how you describe someone who isn't part of the contract, but may have a connection to it.
In the contract where Pat buys surfboards from Alice, Henry is a third party, because he's not part of that sale contract.
The surfboard sale contract might need to mention Henry with something like "Alice agrees to use the third party surfboard delivery service provided by Henry".
In the contract where Alice pays Henry to deliver the surfboards to Pat, Pat is the third party, because he's not a party to the delivery contract.
The contract between Alice and Henry could mention Pat by saying "When delivering surfboards, Henry agrees to contact the third party shop owner twenty minutes before delivery."
In the contract where Dave pays Meegan for using her web hosting service, Debbie is a third party because she's not involved in that contract.
In the contract where Debbie pays Dave for a teapot, Meegan is a third party because she's not involved in that contract.