Checkpoint

A checkpoint is a designated block that a blockchain protocol or its clients treat as a fixed reference point — a block whose place in history is agreed upon and used to anchor finality, speed up syncing, or prevent deep chain rewrites. Instead of evaluating the entire chain from the beginning, nodes can reason from the checkpoint forward.

The most prominent modern use is in Ethereum's proof-of-stake consensus, where the first block of each epoch serves as a checkpoint that validators vote on. When more than two-thirds of staked ether attests to a pair of consecutive checkpoints, the earlier one becomes finalized — meaning the protocol guarantees it can never be reverted without at least one-third of all stake being destroyed. Checkpoints also appear as "weak subjectivity" anchors: a newly syncing node starts from a recent trusted checkpoint rather than verifying years of history, which also defends against long-range attacks that fabricate an alternative ancient history.

Historically, early Bitcoin software shipped with hard-coded checkpoints — block hashes baked into the code — to block certain denial-of-service attacks during initial sync; these were largely retired as the network matured, since developer-chosen checkpoints sit uneasily with the idea that the chain alone defines history. A common misconception is that checkpoints let developers rewrite or freeze the ledger; in proof-of-stake systems they are produced by validator votes under protocol rules, not by any team's decision.