Data Layer
A structured JavaScript object on a web page that passes page, user and event data to tag management and analytics tools.
A planned data layer keeps analytics and ad conversions stable when page designs change, because tags read named values such as order total or plan type instead of copying text from buttons and page elements.
On the order confirmation page, developers run dataLayer.push({event: 'purchase', transaction_id: 'T1045', value: 249.00, currency: 'EUR'}). GTM triggers on the purchase event and passes the values to GA4 and Google Ads tags.
The data layer only reflects what developers push, so missing or renamed keys silently break reporting. It holds data in the browser where users can read it, so it should not contain personal data that the consent setup does not allow.
In Google Tag Manager (GTM), the data layer is a global array called dataLayer, declared before the container snippet. Code adds objects with dataLayer.push(), and a special event key triggers tags; values persist only for the current page. Names must be used consistently across pages. The W3C Customer Experience Digital Data Layer proposed a vendor-neutral digitalData object, and a data layer can also feed Server-Side Tracking.
Performance Advertising, Systems Architecture
Google for Developers — The data layer — https://developers.google.com/tag-platform/tag-manager/datalayer; W3C Customer Experience Digital Data Community Group — Customer Experience Digital Data Layer 1.0 — https://www.w3.org/2013/12/ceddl-201312.pdf