XM usually refers to XM Satellite Radio.
XM Satellite Radio (XM) was one of the three satellite radio (SDARS) and online radio services in the United States and Canada, operated by Sirius XM Holdings.
Extensible Messaging and Presence Protocol (XMPP) is a communications protocol for message-oriented middleware based on XML (Extensible Markup Language).
XMLHttpRequest (XHR) is an API in the form of an object whose methods transfer data between a web browser and a web server.
XMM-Newton, also known as the High Throughput X-ray Spectroscopy Mission and the X-ray Multi-Mirror Mission, is an X-ray space observatory launched by the European Space Agency in December 1999 on an Ariane 5 rocket.
The XM25 Counter Defilade Target Engagement (CDTE) System, also known as the Punisher and Individual Semiautomatic Air Burst System is an airburst grenade launcher derived from the XM29 OICW.
XML-RPC is a remote procedure call (RPC) protocol which uses XML to encode its calls and HTTP as a transport mechanism.