Glossary
Sharding
Splitting a blockchain's state and execution across many parallel partitions (shards) so that capacity scales with the number of shards.
A single-chain blockchain forces every node to process every transaction. Sharding partitions the state and assigns each shard to a subset of validators; cross-shard messages travel via a base chain or via proofs.
Ethereum's original 2018 roadmap centered on execution sharding. The plan has since shifted: execution is outsourced to L2 rollups, and the L1 only shards data — "danksharding" — to give rollups cheap data availability. The thinking is that proving correctness once is better than re-executing in parallel.