A valid block that was not included in the canonical chain due to a competing block at the same height; may still receive partial rewards.
On account-based chains like Ethereum (pre-merge), when two miners found blocks nearly simultaneously, one became canonical while the other was an "uncle" (ommer). Uncles help improve security and decentralization by rewarding near-miss blocks.
"A miner whose block lost the race still received an uncle reward when another miner referenced it, reducing variance in miner income."
All terms and definitions may update as the Cryptionary improves.