Genesis Block
The genesis block is the very first block of a blockchain, hard-coded into the software rather than mined by the network.
In depth
Every chain needs a starting point that all nodes agree on. Bitcoin’s genesis block was created on 3 January 2009 and famously embeds a newspaper headline about bank bailouts. Because nothing comes before it, its "previous hash" field is all zeros.
Why it matters
It is the anchor from which the entire chain — and every balance on it — descends.