DOCX – Office Open XML

Office Open XML (also informally known as OOXML or Microsoft Open XML (MOX)) is a zipped, XML-based file format developed by Microsoft for representing spreadsheets, charts, presentations and word processing documents. The format was initially standardized by Ecma (as ECMA-376), and by the ISO and IEC (as ISO/IEC 29500) in later versions.

Format introduction For most of its history, Microsoft Word used its own format for its saved files, DOC. Since 2007, with an updated version of Word (and Microsoft Office), the default save format has been changed to DOCX. It was not just an overdue "extreme" version of the format of the 1990s — optional X stands for Office Open XML standard.
Technical details Microsoft Office 2010 provides read support for ECMA-376, read/write support for ISO/IEC 29500 Transitional, and read support for ISO/IEC 29500 Strict. Microsoft Office 2013 and Microsoft Office 2016 additionally support both reading and writing of ISO/IEC 29500 Strict. While Office 2013 and onward have full read/write support for ISO/IEC 29500 Strict, Microsoft has not yet implemented the strict non-transitional, or original standard, as the default file format yet due to remaining interoperability concerns.
File extension .docx .docm
MIME application/vnd. openxmlformats-officedocument. wordprocessingml. document
Developed by Microsoft, Ecma, ISO/IEC
Type of format document
Associated programs Microsoft Office, LibreOffice, Kingsoft Office, Google Docs
Wiki https://en.wikipedia.org/wiki/Office_Open_XML