A "docx file" is a type of computer file that contains a document, usually created with a word processing software like Microsoft Word. It stores text, images, and formatting in a way that is easy to open and edit. Full definition
Bob Levine pointed out in InDesign Magazine that Word's DOCX file format is a glorified ZIP file. (bergsland.org)
docx file extension for documents by default, whereas Macs use. (cv-library.co.uk)
The Calibre developers supply a sample docx file with pretty complex formatting which they invite you to use when you try this new feature for yourself. (sherprog.com)