...free and open online access to academic information, such as publications and data.

OA's general definition is literally that simple.

However, the Budapest Open Access Initiative offers a much more detailed definition (for you fans of academese):

...by 'open access' to this literature, we mean its free availability on the public internet, permitting any users to read, download, copy, distribute, print, search, or link to the full texts of these articles, crawl them for indexing, pass them as data to software, or use them for any other lawful purpose, without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself. The only constraint on reproduction and distribution, and the only role for copyright in this domain, should be to give authors control over the integrity of their work and the right to be properly acknowledged and cited.

So, to review...open access means openly-licensed scholarly items that are free and on the internet.  (And yes; here we will acknowledge that not everyone has internet access and/or tools with which to access it, and thus do not have equal access to these openly-licensed, free materials.) More on this particular equity issue later.