Third-party tracking

Definition
Third-party tracking is

"tracking . . . performed by other 'third party' entities, different from the entity the user is directly connected to (the user being the 'second party'), that track the user's browsing activity over time and across different websites. For example, Facebook tracks across sites via its 'Like' button; each time a user visits a site that contains a Facebook 'Like' button, Facebook is informed about it, even if the user does not click on this button."