From Science Europe Data Glossary
Definition: digital data can be assumed to be authentic if it is provable that it has not been corrupted after its creation.
Alternative definition: digital data is authentic if it is what it is claimed to be (by the metadata).
References: http://www.igi-global.com/dictionary/data-authenticity/6684; C. Lynch, "Authenticity and Integrity in the Digital Environment: An Exploratory Analysis of the Central Role of Trust", Authenticity in a Digital Environment, CLIR Reports pub92, pp. 32-50 (Council on Library and Information Resources, Washington DC, 2000), ISBN 1-887334-77-7, http://www.clir.org/pubs/reports/pub92/lynch.html