A set of standardized licences that allow creators to specify how their work (including data) can be used, shared, and modified.
Creative Commons licences range from CC0 (no rights reserved, use for anything) to CC BY (attribution required) to CC BY-NC (non-commercial only) to CC BY-ND (no derivatives). For data, CC0 and CC BY are the most useful because they allow the widest range of uses including commercial applications.