Checkpoint

A checkpoint is a block that a protocol or its client software marks as an agreed, non-revertible point in the chain's history, so nodes will refuse any alternative branch that conflicts with it. Historically some clients shipped hard-coded checkpoints to speed up initial sync and block long-range rewrites of old history. Modern proof-of-stake designs use them as part of consensus: Ethereum's Casper FFG justifies and finalises checkpoint blocks at epoch boundaries through validator votes. Checkpoints shrink the window for deep reorgs, but they introduce a trade-off, because whoever sets them influences which history counts as valid. Developer-imposed checkpoints are therefore controversial, while protocol-level checkpoints agreed by validator vote are considered part of the consensus rules.