Definition
Reindexing is Bitcoin Core's recovery and rebuild operation. Launching with -reindex wipes the block index and the chainstate (UTXO) databases, then rebuilds both by re-reading the raw blk*.dat block files already on disk. Any optional indexes that are active, such as txindex or the coinstats index, are rebuilt too. This is the tool you reach for when the on-disk databases become corrupted, after an unclean shutdown that leaves the node unable to start, or when you switch on a new index that must be backfilled over the whole chain.
reindex vs reindex-chainstate
A lighter variant, -reindex-chainstate, wipes and rebuilds only the chainstate (the UTXO set) from the existing block files, leaving the block index intact. This is faster and useful when only the UTXO database is suspect, but it cannot fix a damaged block index and will not work on a pruned node where the old block files have been deleted. Because a full reindex re-validates the entire chain, it can take many hours and benefits from a temporarily enlarged dbcache.
When to use it
Reindex is a heavy operation, not a routine one. Reach for it after disk errors, file corruption, or to backfill a newly enabled index; reach for reindex-chainstate when only UTXO state looks wrong. Neither rewrites or re-downloads block data from peers, so both depend on the block files being present and intact.
For related rebuild-time tuning, see dbcache and the transaction index (txindex).
In Simple Terms
Reindexing is Bitcoin Core’s recovery and rebuild operation. Launching with -reindex wipes the block index and the chainstate (UTXO) databases, then rebuilds both by re-reading…