Site
Informally, a site is a website, which is a collection of web pages, served from the same domain, and maintained by a single organization.
Browsers sometimes need to distinguish precisely between different sites. For example, the browser must only send SameSite cookies to the same site that set them.
For this more precise definition a site is determined by the registrable domain portion of the domain name. The registrable domain consists of an entry in the Public Suffix List plus the portion of the domain name just before it. This means that, for example, theguardian.co.uk, sussex.ac.uk, and bookshop.org are all registrable domains.
According to this definition, support.mozilla.org and developer.mozilla.org are part of the same site, because mozilla.org is a registrable domain.
In some contexts, the scheme is also considered when differentiating sites. This would make http://vpl.ca and https://vpl.ca different sites. Including the scheme prevents an insecure (HTTP) site from being treated as the same site as a secure (HTTPS) site. A definition that considers the scheme is sometimes called a schemeful same-site. This stricter definition is applied in the rules for handling SameSite cookies.
Examples
These are the same site because the registrable domain of mozilla.org is the same:
https://developer.mozilla.org/en-US/docs/https://support.mozilla.org/en-US/
These are the same site because the port is not relevant:
https://example.com:8080https://example.com
These are not the same site because the registrable domain of the two URLs differs:
https://developer.mozilla.org/en-US/docs/https://example.com
These are the same site, or different sites if the scheme is considered:
http://example.comhttps://example.com