The first block of a blockchain is known as the genesis block. Bitcoin’s current versions number it block 0, although earlier versions numbered it 1. The first block in a blockchain is usually hardcoded into the software that uses it.
It’s unusual because it doesn’t contain a reference to a previous block and thus generates an unspendable subsidy for Bitcoin and most of its descendants.
The first Bitcoin block was called “Genesis” because it was the first block ever mined—hence the name “Genesis Block.” It is the original block in the Bitcoin blockchain and serves as the prototype for all other blocks.
The genesis block was created by Satoshi Nakamoto, the founder of Bitcoin, and is the first in every blockchain that has been subsequently created.
What makes genesis blocks unique is that they have no predecessor block to reference, creating the first such instance of a “single-parent” or “single-point-of-origin” block.
Having no predecessors means that their hashes are unpredictable and not subject to manipulation. This makes them highly reliable as they cannot be changed retroactively without changing all subsequent blocks, which requires overwhelming computational power.