What is Client ID?
A unique, randomly generated string assigned to a specific browser and device combination, used by GA4 to anonymously identify returning users.
When you visit a website with GA4 installed for the first time, a first-party cookie named _ga is dropped in your browser. This cookie contains the Client ID.
Example Client ID: 123456789.1609459200
Limitations
Because it relies on a cookie, the Client ID is tied strictly to the browser. If a user visits your site on their iPhone (Safari) and later returns on their laptop (Chrome), they will have two completely different Client IDs and will be counted as two separate users in GA4.
This is why implementing the User-ID feature (when users log in) is critical for accurate cross-device tracking.
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Related terms.
A unique timestamp parameter generated when a user starts a visit. It groups all the events that occur during that specific visit together.
A unique, persistent identifier that you generate in your own backend database and pass to GA4 to perfectly track authenticated users across devices.
A tracking cookie created and read only by the exact domain the user is currently visiting.