The Berlin Declaration on Open Access to Knowledge in the Sciences and Humanities” (Berlin Declaration) is a foundational document of the Open Access (OA) movement.
The Berlin Declaration was created in 2003 by a number of OA groups and academic institutions, and was intended to build on the Bethesda Statement on OA Publishing and the Budapest Open Access Initiative by presenting practical ways interested parties could “promote the Internet as a functional instrument” for OA. (Source)
The Berlin Declaration presents a goals statement and a definition of OA which are similar to those in the documents it builds upon. It also lays out a number of ways interested parties can contribute to the OA movement, which are vague enough not to be limited by future technological developments and which mostly involve acting as an OA advocate.
You can read the full text of the Berlin Declaration online at the Max Planck Society, as well as sign the declaration yourself. An updated mission statement authored in 2013 is also available on the site.