第三者ドメイン

[1] Creating Safari Content-Blocking Rules (2017-06-06 17:05:24 +09:00) <https://developer.apple.com/library/content/documentation/Extensions/Conceptual/ContentBlockingRules/CreatingRules/CreatingRules.html#//apple_ref/doc/uid/TP40016265-CH2-SW1>

first-party
The rule is triggered only if the resource has the same scheme, domain, and port as the main resource on the page.
third-party
The rule is triggered if the resource is not from the same domain as the main page resource.