Plain-English definition
A rollup bundles many transactions into one, executes them off-chain, and posts the result to the base chain. Optimistic rollups assume honesty and allow challenges; zero-knowledge rollups prove correctness with cryptography.
The difference matters for how quickly you can withdraw and how much you must trust the operator. It is plumbing, but plumbing that holds real money.
Why it matters
Rollup matters because crypto markets can look precise while still being shallow, crowded, or poorly sourced. A useful read is not just the number. It is the number, the size behind it, the context around it, and the part it cannot explain.
Simple example
A simple Rollup example is a reader comparing two venues with the same headline price but different depth. The deeper venue can absorb more capital before the trade, peg, or yield reading starts to move against the user.
How Bathymark uses this term
Bathymark uses Rollup to translate open market data into live liquidity readings. When the term touches a live page, the reading links back to the relevant chain, protocol, stablecoin, DEX, perps, or Signal so the source remains checkable.
Common mistake
The common mistake is treating Rollup as a complete signal by itself. Bathymark treats it as one instrument: useful when read beside liquidity, source, size, and what the number cannot prove.