"a persistent data structure is a data structure that always preserves the previous version of itself when it is modified. Such data structures are effectively

"a persistent data structure is a data structure that always preserves the previous version of itself when it is modified. Such data structures are effectively immutable, as their operations do not (visibly) update the structure in-place, but instead always yield a new updated structure. The term was introduced in Driscoll, Sarnak, Sleator, and Tarjans' 1986 article". It's effectively the same thing. A third name for the same thing is "append only".