Checkpoint
Checkpoint

Checkpoint

Checkpoints are when block hash values up to a specific point in time are hard coded into the official Bitcoin client. The client takes all transactions confirmed up to the checkpoint as irreversible. The client recognizes the blockchain from before the checkpoint as unconditionally true and it is impossible to diverge from anything before the checkpoint. This is a measure to prevent malicious hackers from from altering blocks in the blockchain (such as a 51% attack).