What is Last Irreversible Block (LIB)?

The last irreversible block (LIB) is the last block to have been validated on-chain and cannot be modified or altered. Found on blockchain protocols that utilize an Asynchronous Byzantine Fault Tolerant (aBFT) consensus mechanism, the block confirmation process is two-fold. First, block producers propose what's called an irreversible block with a record of transactions. Then, if a majority of block producers acknowledge the proposed block, it achieves irreversible status, and the transactions are considered validated.