The name, Cross-Site Scripting, may not intuitively relate to its current definition. This is because the term originally referred to a related, but different attack. In the late 1990s and early 2000s, it was possible to read data from web pages loaded in adjacent windows or frames using JavaScript code. Thus, a malicious website could cross the boundary between the two and interact with contents loaded on an entirely different web page not related to its domain. This was later fixed by browser developers, but the attack name was inherited by the technique that makes web pages load and execute malicious scripts in the browser rather than reading contents from adjacent frames.
In simple terms, an XSS attack allows the attacker to execute malicious script code in another user's browser. It could be JavaScript, VBScript, or any other script...
