A future interest is the right to possess a piece of property in the future.
- with a condition precedent before the remainder-holder can take possession or
- one that is given to an unascertained person.
It's like being subject to a condition subsequent, but backwards. Something has to happen before the natural termination of the preceding estate.
e.g. O to A for life, then to B if B has reached 25 years old.
An executory interest automatically takes effect upon the happening of the condition.