Glossary
Definition drift
When copies of the same business definition, held in different systems or documents, stop agreeing with each other.
Most business definitions exist in several places at once: a requirement, an implementation, a wiki page, a report label. Definition drift happens when one of those copies changes and the others do not.
Drift usually comes from a correct local change that nobody else heard about. The effect is that two numbers which should match stop matching, and the reason is hard to find because nothing raised an error.
How it is detected
Detecting drift needs a reference: an approved version of the definition that every other copy can be compared against. Without one, there is no way to say which copy has drifted, only that they differ.
Related ideas
Drift is the trigger for impact analysis, and the reason change control needs to cover definitions as well as systems.