A valid block on the blockchain that is not part of the main chain.
An orphan is a legitimate block that is not part of the main blockchain. It is also called a stale or detached block. There are various factors that could cause orphaned blocks. Key among them include internet lags, the length of a blockchain, block size and the speed of the node housing the blockchain. These factors cause two blocks to have the same timestamp leading to the main blockchain rejecting one block and absorbing the other.
The time-lapse can occur in a situation where two miners in the Bitcoin blockchain, for instance, simultaneously find, solve and propagate a block. Due to the above factors, the two blocks are seen by different nodes at different times.
As more blocks arrive, nodes will add them to the chain they consider valid. The Bitcoin blockchain automatically believes that the longer chain is the valid one. Therefore, one between the two blocks will, despite being accurate, be an orphan (the shorter chain).
Blockchain developers reduce the presence of orphans by factoring in the propagation delay and block time. An orphan is likely to occur if the decentralized platform has a high propagation delay and a short block time. Note that too many competing blocks could also cause forks, hence, the introduction of competing chains. These chains lead to unstable or stuck decentralized networks.