From Science Europe Data Glossary
Definition: open access is the practice of providing (unrestricted) on-line access to scientific information that is free of charge to the end-user and free of most copyright and licensing restrictions.
Explanation: in the context of research and innovation, 'scientific information' usually refers to any peer-reviewed (or otherwise subjected to scientific scrutiny and validation by peers) research publication, primarily academic journals, but also theses, conference proceedings, monographs, and book chapters. In case the scientific information refers to research data (data underlying publications, curated data and/or raw data), the preferred term is open data.
Often distinction is made between green and gold open access. Green open access refers to self-archiving by researchers of their output in an open access repository, gold open access to publishing in an open access journal, for which usually Article Processing Charges (APC) are due.
References: http://ec.europa.eu/research/participants/data/ref/h2020/grants_manual/hi/oa_pilot/h2020-hi-oa-pilot-guide_en.pdf; https://en.wikipedia.org/wiki/Open_access; http://legacy.earlham.edu/~peters/fos/overview.htm
See also: open science