What Is a “Checkpoint” in the Context of Ethereum Finality?

A checkpoint is a block that has been either justified or finalized. The term refers to a specific block that has reached a high level of consensus security.

Checkpoints are used by new nodes joining the network to quickly synchronize and verify the correct chain state, which helps defend against long-range attacks. They provide a trusted, secure point in the chain's history that all nodes can agree upon.