The first block of data that is processed and validated to form a new blockchain, often referred to as block 0 or block 1.
A genesis block refers to the first block in a blockchain and is usually hardcoded into its application’s software. A blockchain has multiple “blocks” (containing validated transactions and recorded activity data) linked together by a metaphorical chain.
Each “block” of a cryptoasset contains referential data for the previous one and derives its value/legitimacy from its predecessor. The genesis block, thus, refers to the first block (Block 0 or Block 1) of a new blockchain, to which all other subsequent blocks are attached.
Genesis blocks have special significance, as they form the very foundation of a blockchain and often contain interesting stories or hidden meanings. For instance, Bitcoin’s genesis block contains the now-famous message "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks" — a reference to the deteriorating financial conditions of that time and the rationale for creating cryptocurrencies like Bitcoin and Ethereum. Bitcoin’s genesis block in 2009 contained 50 BTC.
The Bitcoin genesis block is very intriguing not just for its included message, but also due to the fact that the next block was timestamped nearly six days later (the average time is 10 minutes). The present hypothesis is that the catchy Times headline enticed Satoshi Nakamoto to release Bitcoin in public, but that he actually created the genesis block earlier and altered the timestamp accordingly. After testing his software from Jan. 3, 2009, Satoshi possibly deleted all the test blocks and used the genesis block for his mainnet launch.