Aggregator
A service that searches many trading venues or yield sources at once and routes a transaction to the best terms it can find.
A trade aggregator queries pools across several decentralized exchanges, often splitting one order across multiple routes so that no single pool absorbs the full size, and factors network fees into the comparison. Some also solicit quotes from professional market makers, so an on-chain trade can be filled by an off-chain quote that settles on chain. Yield aggregators do the analogous thing for lending rates, moving deposits toward whichever market currently pays more. Using an aggregator adds a dependency: the routing contract itself becomes part of the transaction and is an additional piece of code that must be sound.
In practice
An aggregator may split one swap across three pools on different exchanges because the combined price is better than routing all of it through the deepest single pool.
The common misunderstanding
An aggregator's displayed quote is an estimate at a point in time; the executed price depends on pool balances when the transaction is finally included in a block.