Agenda 21 is a non-binding, voluntarily implemented action plan of the United Nations with regards to sustainable development. It is a product of the UN Conference on Environment and Development (UNCED) held in Rio de Janeiro, Brazil, in 1992. It is an action agenda for the UN, other multilateral organizations, and individual governments around the world that can be executed at local, national, and global levels. The "21" in Agenda 21 refers to the 21st century. It has been affirmed and modified at subsequent UN conferences.
Read more about Agenda 21: Structure and Contents, Development and Evolution of Agenda 21, Implementation of Agenda 21, Agenda 21 At National Level