Glossary · Mesurer proprement

Third-party cookie

A third-party cookie is a small file dropped by a domain other than that of the page you are visiting, usually by a script or an ad image loaded inside it. It is what allows the same person to be recognised from one site to another — and that is why it is disappearing from browsers.

Cookies, consentement, serveur : mesurer la campagne sans pister la personne. The forty terms of the trade

What it is for

It carried online advertising for years: retargeting, audiences, cross-site measurement. Its gradual extinction in browsers is why measurement is moving towards methods that do not depend on it — the server, and links.

How it is told apart

By the domain that drops it, not by its contents. A cookie dropped by the site you are visiting is first-party, whatever it contains; a cookie dropped by a domain loaded inside the page is third-party. It is the context of the drop that counts.

The classic mistake

Calling “third-party cookie” any cookie dropped by a domain other than your own site. A cookie dropped while you are genuinely on that domain — during a redirect, for instance — is first-party, and browsers treat it as such.

Mesure sans cookie

The same file, two statuses

Third-party

Dropped by a domain loaded in the page. Follows the person between sites.

First-party

Dropped by the domain you are on. Never leaves it.

It is the context of the drop that decides, never the file's contents.

Ce que l’outil collecte, et ce qu’il refuse

What Subtraq does with it

The link drops no third-party cookie. A first-party cookie is placed on the redirect domain, written by the server so unreadable by any page script, and it contains only an identifier for a click already recorded — nothing about the person, nothing about their browsing elsewhere. Its lifetime follows the attribution window, capped at thirty days.