(Electronic PUBlishing) An open standard for electronic books and Web publishing from the International Digital Publishing Forum (www.idpf.org). Introduced in 2007 as the successor to the Open eBook format, EPUB documents are marked up in XHTML or Digital Talking Book (DTBook). OPS, OCF and OPF An EPUB document conforms to three IDPF standards. OPS (Open Publication Structure) defines the markup language. OCF (Open Container Format) is a zipped archive with an .EPUB extension that contains the marked-up publication, related images and an OPF (Open Packaging Format) file that references everything in the OCF, provides meta-data and ties it all together. See Open eBook.