An orphan block, or detached block, is a valid block that is not part of a blockchain. This event occurs when two miners produce valid blocks a similar times. The miner whose block is produced later or that propagates slower ends up with the orphaned block. Other nodes in the network will discard this orphan when they are notified of the more widely accepted block.
These blocks can also be produced when a network is attached by an actor attempting to reverse a transaction.
Orphan blocks are much more common on low block time, low difficulty networks as it is more likely for many miners to find valid solutions and to submit them to the network in a narrow time frame leading to the acceptance of only a single valid block and the rejection of the others.