What is the genesis block?
Bitcoin's first block, mined by Satoshi Nakamoto on 3 January 2009. Every block since links back to it.
Block 0 began the chain. Its only payment carries that day's Times headline, “The Times 03/Jan/2009 Chancellor on brink of second bailout for banks”, a timestamp that also comments on the bank bailouts of the day.
Its 50 bitcoin reward can never be spent: the software skips connecting the genesis block's payment, so it never entered the spendable set. The quirk has been kept ever since.
References
- Bitcoin Core, “ConnectBlock in src/validation.cpp”.