Block expiry refers to the maximum time allocated for a block to remain valid on a blockchain before it is considered outdated. Each block in a blockchain contains transactions that need to be confirmed by network participants. If a block is not confirmed within a specific timeframe, it may be disregarded.

This concept helps maintain the efficiency and security of the network. When blocks expire, it prevents miners from wasting resources on processing stale blocks that no longer reflect the current state of the blockchain. The time allowed for block expiry can vary between different blockchain networks.

Short expiry periods ensure that the chain remains updated, while longer periods might provide flexibility but risk inefficiencies. Implementing block expiry can enhance the speed of transaction confirmation, as nodes are encouraged to work on more recent transactions instead of outdated ones.

Overall, block expiry is an essential feature that helps to keep a blockchain dynamic, ensuring that it reflects the most recent and relevant data.