Permissive Creative Commons licence. Free reuse, modification, and commercial use, with attribution.
CC BY 4.0 is the most permissive of the Creative Commons licences that still requires attribution. It permits any reuse — copying, modifying, redistributing, and commercial use — provided that the original author is credited, a link to the licence is provided, and any modifications are indicated.
Version 4.0 (released November 2013) is jurisdiction-neutral: the same licence text applies internationally. It introduced the explicit covering of database rights (sui generis database rights in EU jurisdictions), which earlier versions handled inconsistently.
For data publishers, CC BY 4.0 is the default recommendation in most international open-data conversations because it is well-understood, internationally-recognised, and compatible with most other open licences.