In Ethereum’s former proof-of-work system, an uncle block was a competing block that did not become part of the canonical chain but whose header could qualify for recognition in a later canonical block. “Ommer” is an alternative, gender-neutral name for the same concept.
Why competing blocks appeared
Mining was probabilistic and network messages took time to propagate. Two miners could build on the same parent before learning about each other’s work. The network eventually continued along one branch, leaving the other candidate outside the accepted transaction history.
Ethereum allowed eligible uncle headers to be referenced under ancestry and inclusion rules. This provided partial compensation for work that otherwise would have earned nothing and reduced disadvantages associated with propagation delays.
Recognition did not confirm its transactions
Referencing an uncle header did not import that block’s transactions into the canonical ledger. A transaction needed separate inclusion in a canonical block to count toward the accepted history. Header eligibility and transaction confirmation were distinct questions.
Uncle blocks describe historical Ethereum mining. Ethereum’s transition to proof of stake removed ommer inclusion and ommer rewards. Old block explorers and mining discussions may still show these records; they are not an ongoing reward category for Ethereum validators.