A file format that is platform-independent and made available to the public without any restriction that impedes the re-use of documents. Source: OD Directive

A file format with no restrictions, monetary or otherwise, placed upon its use and can be fully processed with at least one free/libre/open-source software tool. Patents are a common source of restrictions that make a format proprietary. Often, but not necessarily, the structure of an open format is set out in agreed standards, overseen and published by a non-commercial expert body. A file in an open format enjoys the guarantee that it can be correctly read by a range of different software programs or used to pass information between them.

Source: ODH