Sequencer
The party that receives layer 2 transactions, decides their order, and packages them for posting to the base chain.
The sequencer is what makes a rollup feel instant: it acknowledges a transaction within moments, long before the batch reaches the base chain, and users treat that soft confirmation as good enough for everyday use. Nearly all rollups run a single sequencer operated by the development team, so an outage halts new transactions and the operator has the technical ability to reorder or exclude them, which is why designs include a forced-inclusion path through a base-chain contract after a delay. Fees and ordering value collected by the sequencer are usually a rollup's main income, and while dashboards may call this revenue, it is fee income from operating a network rather than the sale of a product, and it is reported gross of the data and proving costs it must cover. Shared and decentralized sequencer designs aim to remove the single operator, and are at varying stages of deployment.
In practice
Rollup sequencer outages have stopped new transactions from being accepted for hours while the underlying base chain continued producing blocks normally.
The common misunderstanding
A sequencer on a proper rollup cannot steal funds or forge state; its power is over ordering, timing and inclusion, not over what counts as valid.