Open data is data that can be freely used, re-used and redistributed by anyone - subject only, at most, to the requirement to attribute and sharealike. (Open Knowledge Foundation, 2022)
The full Open Definition gives precise details as to what this means. To summarize the most important:
If you're wondering why it is so important to be clear about what open means and why this definition is used, there's a simple answer: interoperability.
Interoperability denotes the ability of diverse systems and organizations to work together (inter-operate). In this case, it is the ability to interoperate - or intermix - different datasets.
(Open Knowledge Foundation, 2022)
Eugenia Loría-Solano, Juliana Elisa Raffaghelli. (2021). Competency model for open data literacy in professional learning within the context of Open Government Data (OGD). Proceedings of the Doctoral Consortium of Sixteenth European Conference on Technology Enhanced Learning. [Summary] http://ceur-ws.org/Vol-3076/ECTEL2021_DC_paper13.pdf,
Antti Poikola, et.al.. (2022). The Open Data Handbook. Open Knowledge Foundation. [Summary] https://opendatahandbook.org/guide/en/