Crypto glossary
Layer 0
Layer 0 describes infrastructure beneath layer 1 blockchains that lets many chains launch, share security or communicate, such as Polkadot's relay chain.
Layer 0 is a loose label for the foundations on which other blockchains are built. It can mean a hub that coordinates and secures several chains, like Polkadot's relay chain, or a toolkit plus messaging standard that lets independent chains be launched and connected, like the Cosmos SDK with IBC. Avalanche's framework for launching custom L1s is often placed in the same category.
The term is popular in project marketing because it signals that a network aims to host an ecosystem rather than compete as a single chain. For readers it is useful to group platforms whose main product is the ability to create and connect other chains, and to distinguish them from general-purpose L1s like Ethereum or Solana.
There is no strict definition, and the layer numbers do not describe the same thing on every network. Some writers also use layer 0 for the physical network and peer-to-peer layer. It should not be confused with LayerZero, the name of a specific cross-chain messaging protocol.
Example
A team wanting its own chain with built-in links to sister chains compares layer 0 options: buying coretime on Polkadot or launching a Cosmos SDK chain connected by IBC.