A more thorough explanation:

Definition: Obligatio naturalis is a term used in Roman law to refer to an obligation that is not legally enforceable, although it may produce legal effects. It is an obligation that derives only from the law of nature.

Example: An example of obligatio naturalis is when a person voluntarily pays a debt that is already time-barred. The person is not legally obligated to pay the debt, but their payment may produce legal effects, such as preventing the creditor from harassing them for the debt.

This example illustrates the definition of obligatio naturalis because the person is not legally bound to pay the debt, but their payment is still recognized as having legal effects. The obligation to pay the debt is derived from the natural law of fairness and equity, rather than from a legal contract or obligation.