From Science Europe Data Glossary
Definition: data sharing is the re-use of data by someone else than who collected the data.
Alternative definition: data sharing is the practice of making data, including metadata and additional data documentation (e.g. on methods, techniques and procedures) that allow data to be correctly interpreted, available to others for scholarly research.
Explanation: the general concept of data sharing is grounded in the assumption that data are a valuable long-term resource and that sharing them and making them publicly available is essential if their potential value is to be realized. Data sharing requires the systematic collection, curation and dissemination of data. The re-use of research data is increasingly promoted or regulated in data policies by research funding and performing organisations, by scholarly journals, and is sometimes stipulated in codes of conduct for researchers, because transparency and openness are considered to be an important part of scientific discovery. A European Directive on the re-use of public sector information in general provides a common legal framework for a European market for government-held data (public sector information).
References: http://ec.europa.eu/digital-agenda/en/european-legislation-reuse-public-sector-information; http://esciencelibrary.umassmed.edu/thesaurus/data-sharing
See also: secondary data; data availability policy