The first block of a blockchain after it launches on a mainnet - independent blockchain computer network. As such, it is the only data block without a preceding block, yielding a mining reward that is not spendable.
When any blockchain launches, its chain has to start from the beginning. This block 0 forms the foundation of the blockchain, to which all other subsequent blocks are added to form a chain. Because all blocks store a reference to the previous one, the Genesis Block is unique in that it has no such reference.
In other words, the hash value of Genesis Block is set to value 0. Keep in mind that cryptographic hash in blockchain terminology translates to a block's digital fingerprint. Therefore, when the hash value is 0, it signifies there was no data prior to the Genesis Block.
In contrast, subsequent blocks will have sequential values, from 1 and onward, with each block's hash referencing the previous one. By the same token, the Genesis Block hash is added to every new transaction within a new block, until all blocks are mined.
In practice, we can take a look at what this means for the world's most popular cryptocurrency - Bitcoin. As long as its network of nodes exists, there will forever be a message embedded within the Bitcoin's Genesis Block - "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks"
Image Source: Bitcoin Wiki
Bitcoin's creator, pseudonymous Satoshi Nakamoto, chose this message as a way to "christen" Bitcoin and its purpose. Not only does it frame the cryptocurrency as an alternative to central banking, which resulted in banking bailouts, but it proves there were no Bitcoins pre-mined before that date.
Although blockchains could exist without Genesis Blocks, they infuse the protocol with trust. Otherwise, miners would be able to start validating blocks at any point in the chain, without having the Genesis Block as the reference point.
Each block in the blockchain is differentiated by height, order, timestamp, position, and mining difficulty. Given that block height represents the number of blocks before it, Bitcoin's Genesis block looks like this:
- Number of transactions: 1
- Transaction fee: €0.00
- Block height: 0
- Block reward: 50 BTC
- Timestamp: 2009-01-03 23:45
- Nonce: 2,083,236,893
- Block difficulty: 1
Nonce refers to the "number used only once," as the number that Proof-of-Work miners have to unlock to validate it and receive the mining reward.