A more thorough explanation:

Definition: In civil law, onerous cause refers to an advantage gained in return for fulfilling a contractual obligation.

For example, if a person agrees to pay rent in exchange for the right to live in an apartment, the right to live in the apartment is the onerous cause. Similarly, if a company agrees to provide a service in exchange for payment, the payment is the onerous cause.

The onerous cause is an essential element of a contract, as it ensures that both parties receive something of value in the exchange. Without an onerous cause, a contract would be considered gratuitous, meaning that one party is giving something without receiving anything in return.