A definition of plain language was adopted by the International Plain Language Working Group (IPLWG), a joint project between Clarity, the Centre for Plain Language and PLAIN.
This project was established to investigate the future direction for plain language as a profession.
The IPLWG definition:
A communication is in plain language if its wording, structure, and design are so clear that the intended readers can easily find what they need, understand what they find, and use that information.
The IPLWG will make the definition available in other languages soon.