Reversion Definition:

A future interest left in a transferor or his (or her) heirs.

A reservation in a real property conveyance that the property reverts back to the original owner upon the occurrence of a certain event.

For example, Jim gives Bob a building 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.