Shadow Information

Shadow information is any data that objects need to maintain, above and beyond their normal domain data, to persist themselves. This typically includes primary key information, particularly when the primary key is a surrogate key that has no business meaning, concurrency control markings such as timestamps or incremental counters, and versioning numbers. For example an Order object may have shadow information of an OrderId.

http://www.agiledata.org/essays/mappingObjects.html#ShadowData