A genesis block is the first block used to start a blockchain. In Bitcoin, it is known as block 0.
Bitcoin’s first block
Bitcoin’s developer glossary defines the genesis block as the first block in the blockchain. Because block height counts the number of blocks that precede a given block, the genesis block has height zero: no earlier block precedes it.
Bitcoin’s developer guide likewise identifies the first Bitcoin block as block 0, commonly called the genesis block. The term describes its position at the beginning of that particular blockchain.
Is the definition universal?
The basic idea is widely used, but details can differ between networks. Ethereum’s glossary describes a genesis block as the first block used to initialise a particular network and its cryptocurrency. That wording is a useful reminder not to assume that every network has Bitcoin’s numbering, contents or configuration.
In short
In Bitcoin, the genesis block is the first block, called block 0, and has height zero. It is the starting block for that blockchain, not the same thing as the first transaction in a block.