Glossary
Uncle Block (Ommer)
On pre-Merge Ethereum, a valid block whose parent was the same as the canonical chain's tip but which lost the propagation race — partially rewarded.
Pre-Merge Ethereum had short block times (~13s), which produced relatively many near-simultaneous valid blocks. The chain canonicalized one and rewarded the loser as an "uncle" (also called "ommer" to avoid gendered terminology). Uncles received a fraction of the full block reward.
Uncles existed mostly to keep mining-pool centralization low: small miners couldn't be steamrolled by faster propagation from large pools. The concept disappeared at the Merge — PoS has deterministic slot-based block production, so there are no propagation races.