A visitor ID is an identifier assigned to an anonymous visitor, usually stored in a cookie, that ties their activity together before any hard identifier, such as an email address, exists.
Category
Identity
Also known as
anonymous ID; cookie ID
It is usually the soft ID a CDP relies on to build an anonymous profile, and its fragility is the whole story: browser privacy like ITP caps how long it survives, and it can also reset when cookies expire, a user clears them, or consent changes, so a returning visitor can be issued a new one and look brand new. That is exactly what custom tracking domains and server-side setups try to protect.
In practice
Assume visitor IDs are short-lived on privacy-restricted browsers and design for the reconnection. The goal is to link a visitor ID to a hard ID as soon as the person identifies, so the earlier anonymous behavior is not stranded when the cookie resets. In Bloomreach, thesoft ID / hard ID manualwalks through exactly how this stitching works.