"A piece of data or content is open if anyone is free to use, reuse, and redistribute it — subject only, at most, to the requirement to attribute and/or share-alike." - from https://opendefinition.org/
Open data often refers to data made public by various levels of governments, but can also refer to scholarly research data or datasets shared by NGOs and other organizations.
Open data blogger and PhD student Tim Davies has compied definitions of related terms, such as big data, raw data, and linked data.