What are orphan blocks and why do they matter?

An orphan block is a valid block that the network discards because another block was accepted at the same height first. When two miners produce a block at nearly the same time and the network has not yet fully propagated either one, both blocks are technically valid — but a blockchain can only follow one chain, so the other block is abandoned. The discarded block represents real computational work that contributed nothing to the final ledger. This matters because wasted work means the network's total mining power is less effective at securing the chain than it appears, creating a security gap.