One who, or that which, reverts.
"The old estate reverted to my grandmother's family after the long line of heirs died out without children."
undergo reversion, as in a mutation
"The DNA sequence didn't just stay mutated; it reverted back to its original form after the cell repaired itself."
To turn back, or turn to the contrary; to reverse.
"After months of progress toward peace, tensions caused the fragile agreement to revert quickly."