Orphan Block

Orphan blocks, also referred to as detached blocks, are confirmed blocks. That lose out in the race, often by just a few seconds, to become the canonical block for a blockchain. Blocks are orphaned routinely during the course of Proof of Work mining operations since acceptance of blocks by the network’s nodes does not occur instantaneously.

Meaning, Miners can and do find valid block hashes around the same time. Causing a temporary fork of the blockchain as nodes signal for different blocks since both are valid.

Eventually the network comes to consensus on the canonical block for the chain. Majority rules, whichever block gets the most signals (i.e votes) from nodes wins. Competing blocks, although valid, are orphaned.

Orphan blocks can also occur as a result of attacks on a network. Since transactions are only considered finalized after 6 block confirmations.

When a malicious actor attempts to reverse previously confirmed transactions on a chain and the chain realizes the blocks are invalid thwarting the attack. These blocks are discarded and orphaned in the same way unsuccessful candidate blocks are.

Further Reading Orphan Blocks

Orphaned Blocks Via Bitcoin Block Explorer