As used in this act:

    (a) "Terminable interest" is a possessory or ownership interest in real property which is subject to termination by a provision in a conveyance or other instrument which either creates a right of reversion to a grantor or his heirs, successors or assigns or creates a right of entry on the occurrence of a specified contingency.

    (b) "Specified contingency" is the event described in a conveyance or other instrument creating a terminable interest, the occurrence of which requires or permits the divesting of the terminable interest.