* ESXi: This is the most common meaning. It stands for ESX Server, hypervisor, a type of software used to virtualize servers.
* Enterprise Server X: This was a name for a specific line of servers from IBM.
* Extensible Stylesheet Language - Extensible: This is a less common meaning but could be encountered in the context of web development.
To determine the correct meaning, you need more context. For example, if you are referring to a server operating system, then ESXi is the most likely meaning. If you are talking about a web development technology, then Extensible Stylesheet Language - Extensible might be the correct interpretation.