In a transfer of property where an interest therein is created in favour of a person to take effect only on the happening of a specified uncertain event, or if a specified uncertain event shall not happen, such person thereby acquires a____________.

As per the Transfer of Property Act, 1882 S.21 . Contingent interest .mdash;Where, on a transfer of property, an interest therein is created in favour of a person to take effect only on the happening of a specified uncertain event, or if a specified uncertain event shall not happen, such person thereby acquires a contingent interest in the property. Such interest becomes a vested interest, in the former case, on the happening of the event, in the latter, when the happening of the event becomes impossible.