Cross-domain tracking
Keeping one visitor and session intact when a journey crosses related domains, so checkout does not look like a brand-new referral.
See it
What it is
Cross-domain tracking preserves a visitor and session when they move between related domains you control. Cookies belong to a domain, so example.com cannot simply read the analytics cookie from example-pay.com. A linker passes the needed analytics identifier on an allowed link, and the receiving site adopts it instead of inventing a new visitor.
Reach for it when a marketing site, app, booking flow, or checkout lives on a different registrable domain but belongs to one measured journey. Google Analytics uses a linker parameter named _gl for this job. Without the handoff, the second domain can start a new session and report the first domain as a referral.
Gotcha: both domains must participate, and the destination list must be tightly allowed. Redirects, link shorteners, or code that strips query parameters can break the handoff. Do not put account ids, email addresses, or authentication tokens in the linker. For a hosted payment page you cannot configure, such as Stripe Checkout or a Shopify checkout on its own domain, record the confirmed conversion from its server webhook instead of trying to smuggle identity through the browser.
Ask AI for it
Configure GA4 cross-domain measurement across [domain A] and [domain B] using the same Measurement ID and Google tag linker settings. Allow only those two domains, decorate navigation between them with the _gl linker parameter, and verify neither owned domain is recorded as a referral. Add a browser test that starts on domain A, follows the real link to domain B, verifies _gl was carried, and confirms the journey is not split by a self-referral. Do not pass login tokens, email addresses, or internal user ids in the URL.