Uncle Block
A validly mined Ethereum block that lost a brief race to be included in the canonical chain, but was still partially rewarded for contributing to network security.
An uncle block (sometimes called an "ommer") was, under Ethereum's former proof-of-work consensus, a validly mined block that arrived at nearly the same time as another competing block at the same height, lost the brief race to become part of the canonical chain, but was still referenced by a later block and given a partial block reward. This differed from a simple orphan block (see Kaavon's "Orphan Block" glossary definition) in other proof-of-work chains, which typically receives no reward at all once discarded.
The uncle-block reward mechanism existed specifically to reduce the disadvantage faced by miners with slightly slower network propagation, and to strengthen the chain's overall security by still crediting work that didn't end up on the canonical chain rather than discarding it entirely — this partially offset the tendency for naive proof-of-work systems to favor better-connected miners during blocks produced in quick succession. Uncle blocks became obsolete for new blocks following Ethereum's transition from proof-of-work to proof-of-stake consensus, since the concept is specific to proof-of-work block competition.
Examples
- Under Ethereum's former proof-of-work consensus, a miner whose valid block lost a close race to another miner's block could still receive a partial reward if it was included as an uncle.
- Unlike a simple orphan block in many other proof-of-work chains, an Ethereum uncle block was specifically rewarded, partially compensating for the chain's natural block-propagation delays.