Ouroboros is the consensus algorithm used by the Cardano blockchain to validate blocks.
Ouroboros is a proof-of-stake protocol developed for Cardano. Instead of relying on miners competing with computational work, it organizes time into slots and epochs, with selected stake pools responsible for producing blocks in particular slots.
The protocol was developed by researchers associated with the University of Edinburgh and Input Output Global. Its name comes from the ancient symbol of a serpent or dragon eating its own tail. Cardano’s network uses ADA for staking and transaction fees.
Ouroboros is not the same as Ethereum’s proof-of-stake system, even though both avoid Bitcoin-style energy-intensive mining. Cardano’s staking design also differs from delegated proof-of-stake systems used by some other blockchains, so the protocol name is the most precise answer.