Also known as “checkpoint lockin”. A very deep block (several days old) accepted by the overwhelming majority of users. Reorganization will not occur past that point. Protects most of the history from a 51% attack. Since the checkpoints affect how the main chain is determined, they become part of the protocol and must be recognized by alternative nodes.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc