Identity Merge
Advanced PronunciationEN/aɪˈdentɪti mɜːrdʒ/
Identity merge is the step where an analytics system joins an anonymous profile and a known user profile into one, so pre-login and post-login behaviour belong to the same person.
The trigger is the first event that carries both an anonymous identifier and a user ID. Some tools resolve it on the client with an alias call at login, which is fragile when events arrive out of order; others run a retroactive server-side merge that stamps the user ID onto earlier anonymous events. Either way the merge rewrites history, so an acquisition source or a funnel computed before the merge can change afterwards. The dangerous case is a shared device: without a reset at logout, the next person is stitched into the previous profile.
FAQ
An alias is one explicit client instruction that declares two identifiers to be the same at a point in time. The merge is the outcome the system reaches, and in current tools it usually runs server-side and retroactively, so earlier events are corrected too.