Shifting-executory-interest meaning

(law) A third party interest in an estate in land created by the conditions of a grant wherein the grantor gives the land to a second party, but with the occurrence of a condition divesting the second party of the land in favor of the third party.

Joe gives land "to Mike, but if Harry graduates from law school, to Harry". Harry has an shifting executory interest because his graduation from law school divests Mike of ownership.