Genesis Block
The very first block of a blockchain, created when the network starts and written directly into the software.
Because it has no predecessor, the genesis block cannot reference an earlier hash, and it is hard-coded into every client so all nodes start from the same point. It also fixes the initial state, including any balances allocated before the network opened, which is where pre-mines and foundation allocations are recorded. Two networks running the same software but different genesis blocks are different networks and cannot exchange blocks. Readers meet the term in chain histories, in client configuration files, and in discussions of how a project's initial supply was distributed.
In practice
Bitcoin's genesis block, created in January 2009, contains a newspaper headline in its coinbase field, which is why it is often cited as a timestamp of the network's start.
The common misunderstanding
The genesis block is not mined in the ordinary way; it is written into client software, and any balances it contains are a design decision rather than a result of mining.