Orphan Block

An orphan block is a valid block that was mined and briefly part of a blockchain but ended up excluded from the final chain, usually because two miners found competing blocks at nearly the same time. When that happens, the network temporarily splits, with some nodes building on one block and some on the other; whichever branch gets extended first wins, and the losing block is abandoned along with its miner's reward.

For example, if miners in Asia and North America each solve a Bitcoin block seconds apart, both blocks propagate through different parts of the network. Nodes accept whichever they see first, but once the next block arrives and extends one of the two, every node converges on that longer chain. The transactions in the abandoned block are not lost — those not already included in the winning branch simply return to the mempool to be confirmed in a later block.

Strictly speaking, Bitcoin developers call these "stale blocks," reserving "orphan" for blocks whose parent is unknown, but in everyday usage "orphan block" means the discarded competitor. A common misconception is that orphan blocks indicate an attack or malfunction; they are a normal, occasional byproduct of network latency in any proof-of-work system. Their existence is why exchanges and merchants wait for multiple confirmations — each additional block makes it exponentially less likely a transaction sits on a branch that will be abandoned.