The Open Definition is a document published by the Open Knowledge Foundation (OKF) (previously Open Knowledge International) to define openness in relation to data and content. It specifies what licences for such material may and may not stipulate, in order to be considered open licences. The definition itself was derived from the Open Source Definition for software.
OKI summarise the document as:
Open means anyone can freely access, use, modify, and share for any purpose (subject, at most, to requirements that preserve provenance and openness).
The document is available under a Creative Commons Attribution 4.0 International License, which itself meets the Open Definition.