A cross-site cookie is a browser cookie that can be included in requests made between different sites, rather than only on the site that originally set it. In practice, this usually requires the cookie to be marked with SameSite=None and Secure, and to have a domain scope broad enough for the intended subdomains or related services.
Cybersecurity teams care about cross-site cookies because they enable measurement, attribution, and session continuity across web properties, but they also increase tracking and correlation risk. Adtech, single sign-on flows, embedded content, and some fraud-detection systems rely on them. Attackers can abuse weak cookie scoping or overbroad sharing to link activity across contexts, while defenders look for tight domain boundaries, minimal lifetime, and correct SameSite settings to reduce unwanted cross-site reuse.