A future interest left in a transferror or his (or her) heirs.
A reservation in a real property conveyance that the property reverts back to the original owner upon the occurence of a certain event.
For example, Jim gives Bob a bulding using the words "to Bob for life". Upon the death of Bob, the property reverts back to Jim or to Jim's heirs.
Differs from a remainder in that a remainder takes effect by an act of the parties involved.
A reversion takes effect by operation of the law.
Nor is a reversion a "left-over" as is a remainder. Rather, it reverts the entire title and property.