Glossary
What is a gclid?
The gclid (Google Click Identifier) is a URL parameter that Google Ads appends to your landing-page URL when auto-tagging is enabled. It uniquely identifies the ad click, so both Google and your own systems can tie later conversions back to the exact campaign, ad group and ad that produced the click.
With auto-tagging on, every ad click lands with ?gclid=… in the URL. Google’s own tags store the value in a first-party cookie (typically _gcl_aw) so that a later conversion in the same browser can reference it.
The gclid is central to offline conversion imports: send Google Ads a conversion together with the gclid of the originating click, and it is attributed as if it had been measured online. That is how sales closing in a CRM, by phone or days later still get credited to the ad. It is also how server-side setups report conversions the browser tag missed. On some iOS traffic Google uses the gbraid/wbraid parameters instead, which serve a similar purpose in aggregate form.
Like every click ID, the gclid is exposed to signal loss: cookies get purged and journeys hop between devices. Capturing it at landing and storing it server-side against a durable visitor identity keeps click-to-order attribution intact well beyond browser cookie lifetimes.