Open access (OA) is a publishing movement aimed at making research freely available online.
When research is fully open access, it is free for the public to access, read, download, copy, share, or use for any other lawful purpose. This approach to open access was established in the Budapest Open Access Initiative (2002) and Berlin Declaration (2003).
In general, we say that any research that is free to read online is open access. There are different subtypes of open access, including Gold OA and Green OA:
Gold OA is when research is available as open access immediately from the publisher. Authors usually retain copyright and the publisher may licence the work under a Creative Commons licence.
This model often requires the researcher, institution, or funding body pay a fee to the publisher in exchange for making it open access.
For more on Gold OA, see "Making your research open access: Publishing Gold OA"
Green OA, also called the 'author self-archiving' model, involves making an approved version of a publication openly accessible in a repository, such as Deakin Research Online (DRO).
Most journals allow the author's accepted manuscript (or pre-print) of an article to be made OA in a repository, often after an embargo period. Book publishers tend to have more restrictive policies and often do not allow Green OA deposit.
For more on OA in DRO, see "Making your research open access: Depositing for Green OA"