Photo credit: Tom Magliery 

According to Peter Suber, OA godhead, open access literature is:

1. digital
2. online
3. free of charge (gratis)
4. free of most copyright and licensing restrictions (libre)

Richard Stallman characterized points three and four thusly: "Think free as in speech, not free beer." Ideally, OA is both, although publisher embargoes complicate the picture somewhat. NIH-funded studies, for example, provide for six months of exclusivity before mandating the deposit of research in PubMed Central.

In short, a one sentence takeaway is that "open access stands for unrestricted access and unrestricted reuse."