Definition
Assume Valid, set with the -assumevalid option, is a Bitcoin Core feature that lets a node skip the most expensive part of validating old blocks, namely the verification of every digital signature and script, for blocks that came before a specific hard-coded "known to be good" block hash. It exists to make initial block download dramatically faster without weakening the security model.
How it speeds up sync
A large share of the time spent syncing a new node from scratch goes into checking signatures on historical transactions. The assume-valid block is a recent, deeply buried block whose hash ships inside the Bitcoin Core software in chainparams. For ancestors of that block, the node still checks that the data hashes correctly, that no coins are created from nothing, and that no coin is spent twice; it only skips re-verifying signatures that have already been validated by countless other nodes. Recent blocks, those not yet buried under sufficient proof of work, are always fully validated.
It is not a checkpoint
Crucially, assumevalid has no influence on consensus. If you point it at a block with an invalid history, the node will still reject an invalid chain once it does full validation, and you can disable the feature entirely with -assumevalid=0 to verify every signature from genesis. This is what separates it from old-style checkpoints: it is a performance optimization you can audit or switch off, not a trust anchor baked into the rules.
For sovereign operators who want maximum verification, running with -assumevalid=0 checks the entire history. See also the Address Manager.
In Simple Terms
Assume Valid, set with the -assumevalid option, is a Bitcoin Core feature that lets a node skip the most expensive part of validating old blocks,…