A multi-chain protocol where independent blockchains (parachains) share security and interoperate through a central relay chain.
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Polkadot is a multi-chain protocol launched in 2020 by Gavin Wood, an Ethereum co-founder who also created the Solidity programming language. Polkadot's design centers on a relay chain that coordinates a set of independent chains called parachains. Each parachain has its own logic, governance, and tokenomics, but inherits security from the relay chain's validator set.
The Polkadot relay chain selects validators (Nominated Proof-of-Stake, where DOT holders nominate trusted validators) who secure the entire network. Parachains lease slots on the relay chain through periodic auctions, paying with locked DOT. Cross-chain messages (XCM) let parachains move tokens and call contracts on each other natively. Substrate, the framework Polkadot is built on, is widely used to build parachains and standalone chains.
Polkadot hosts dozens of parachains spanning DeFi (Acala, Hydration), smart contracts (Moonbeam, Astar), privacy (Manta), and identity. Some former parachains have migrated to standalone chains. The Substrate framework also powers chains outside Polkadot's security model. Kusama, Polkadot's "canary network," runs the same software with looser governance for testing.
Polkadot's parachain auction model created early friction (slot prices were high, leading to complex crowd-loan structures) and has been simplified over time. The ecosystem is technically sophisticated but has lagged Ethereum and Solana in user-facing application adoption. Substrate as a framework is widely respected.
Multi-chain bridging and cross-chain communication are also covered in the crypto bridge entry.