Uncle Block

An uncle block — called an ommer in more neutral terminology — is a valid block that was produced at almost the same time as another and lost the race to become part of the main chain. During Ethereum's proof-of-work era, such blocks could still be referenced by later blocks and earn their miner a reduced reward, which cut the advantage of large, well-connected mining pools and lowered wasted work. Uncles appear when block times are short relative to how long a block takes to propagate across the network, so two miners solve at once. They are not failures or attacks: transactions in an uncle simply were not applied and normally get included in a later main-chain block.