A more thorough explanation:

Definition: Third person refers to someone who is not directly involved in a legal case, agreement, or other transaction, but may still be affected by it. This person is also known as a third party or outside party.

Examples:

• In a divorce case, the couple getting divorced are the first and second parties. Any other person involved, such as a child custody evaluator, would be considered a third person.
• When two companies enter into a contract, a third party may be brought in to provide services or goods related to the contract.

These examples illustrate how a third person is not directly involved in the main transaction or agreement, but may still have a role to play or be impacted by the outcome.