It is called a "condition," when something is given on an uncertain event, which may or may not come into existence.

Explanation

(English-origin maxim.) A condition in law is a provision tied to an uncertain future event — until the contingency occurs or fails, the rights dependent on it remain in suspense.

Where this applies

• A father promises to give his son a car if the son graduates from college — the gift is conditional on an uncertain future event that may or may not happen.
• A will leaves a house to a niece on the condition that she marries before age 30 — the bequest depends on an uncertain event.
• An insurance policy pays out only if the insured's house catches fire — the obligation is conditional because the triggering event is uncertain.