AJAX
↔ (process) to Asynchronous Data Fetch from Server (via clunky legacy XHR methods)
aka AJAX, Asynchronous-JavaScript-And-XML, AJAX-Call
XHR
Source: XHR (XMLHttpRequest) - MDN Web Docs Glossary: Definitions of Web-related terms | MDN
A TCP/IP socket connection is established between Client & Server
References
XMLHttpRequest
↔ legacy Interface for old-school AJAX