An uncle block is explained in context under orphaned block, which covers blocks that end up outside a blockchain's main chain, including Ethereum's uncle or ommer blocks. This page explains the specific term and why it has two names.
On Ethereum under proof of work, two miners sometimes found valid blocks at almost the same time. Only one could become part of the main chain, and the other was left aside. Because Ethereum produced blocks far more often than Bitcoin, this happened frequently. Instead of simply discarding the losing block, as Bitcoin does with a stale block, Ethereum let later blocks include a reference to it. The miner of that uncle received a reduced block reward, and the miner who included it earned a small bonus. The design, inspired by research on a protocol known as GHOST, reduced the advantage of large mining pools with fast connections and counted the uncles' work towards the chain's security.
The name comes from a family-tree analogy: the block is a sibling of a parent of the block that refers to it, which makes it an "uncle". Ethereum developers later adopted "ommer", a gender-neutral word for a parent's sibling, and it appears in the protocol's technical specification. The two names mean the same thing. Uncle blocks ended with the Merge in September 2022, since proof of stake on the Ethereum blockchain assigns each slot to a single validator and has no ommer rewards.