Definition
Orphan blocks (more accurately called stale blocks) occur when two miners find valid blocks at approximately the same time. The network temporarily has two competing chain tips. As subsequent blocks are mined, one chain will grow longer and be accepted as the canonical chain, while the other block becomes orphaned.
Miners who produced orphan blocks do not receive the block reward. The transactions in the orphaned block are returned to the mempool and typically included in future blocks. Orphan blocks are a natural and expected part of Bitcoin’s operation.
In Simple Terms
A valid block rejected by the network because a competing block at the same height was accepted first.
A valid block that is not part of the main blockchain because another block at the same height was accepted by the network first.