Open Access is immediate, free, online availability of published scholarly output, with or without permission for free reuse.
Immediate means concurrent with publication. Delayed access, say after a 12- or 24-month publisher embargo, is good, but the impact and public benefit of research suffer the longer it is kept from being fully open.
Free means without cost to the end user or their intermediaries (like schools, hospitals, or libraries). Some open access publishers charge a fee to authors to cover the costs of the infrastructure that makes this possible.
Online availability is the linchpin of open access, enabling the immediate and free sharing of information worldwide.
Published scholarly output traditionally means journal articles, but movements for open monographs, textbooks, datasets, and other types of research and educational materials are gaining strength.
Reuse rights allow for groundbreaking new methodologies, like text and data mining, to be applied to the work. Some publishers and advocacy organizations consider this an indispensable requirement of open access, and some do not.