There is no strict definition i think
- If you have a Stretched cluster or mirrored Storage you have 2 sites
- It can be a different location. One part of the infrastructure is here, other parts are there. Customer often have a "Recovery Site". So we use the term "site" mostly as "location".
Site definition in vSphere environment just works with SSO domain in the Platform Service Controller especially whenever you want to deploy more PSC server (in external PSC mode) or additional VCSA . I think site name gives us a concept of physical site for replication and site recovery solutions like SRM or VR.