Layer-0
The base network layer that lets independent blockchains pass messages and spin up their own connected chains.
Where Layer-1 is a single blockchain like Ethereum, Layer-0 is the framework beneath several of them. Cosmos and Polkadot are the standard examples. Each provides a shared security or messaging backbone, plus tooling so developers can launch an application-specific chain that talks to the rest of the network.
The pitch is interoperability without bridges bolted on after the fact. The catch is that these ecosystems are smaller than Ethereum's, and a chain inheriting shared security also inherits the base layer's failures. The term is more marketing than precise engineering.
Related terms
The base blockchain itself, where transactions settle and security lives, before any add-on networks built on top.
ParachainAn independent blockchain that runs in parallel and leases security from the Polkadot relay chain through a connecting slot.
InteroperabilityThe ability of different blockchains to exchange data and value without relying on a central intermediary.
Cross-ChainDescribes activity that spans two or more separate blockchains, such as moving assets or messages between them.