Glossary
Sequencer
The component of a rollup (or app-chain) that orders incoming transactions and produces L2 blocks before they are posted to L1.
Most rollups today run a single sequencer operated by the team: it orders user transactions, executes them, and periodically posts the resulting state and data to L1.
That centralization is the main caveat in current rollup security: a malicious sequencer can censor or reorder transactions, though it cannot forge state on a fraud-proven or validity-proven rollup. Decentralized sequencing — shared sequencers, rotating leader sets, based sequencing on L1 — is an active area of research and rollout.