A smart contract is a program deployed on a blockchain that runs automatically when its conditions are met, with no intermediary, and whose outcome can be verified by every participant in the network.
It is not a contract in the legal sense: it is code that holds assets and applies rules (transferring a token, releasing a payment, recording a vote). Smart contracts are the basic building block of DeFi, NFTs and DAOs. They are written in dedicated languages: Solidity for Ethereum and compatible networks, Rust for Solana, Move for Aptos and Sui.
Once deployed, a smart contract is in principle immutable, and it often handles real money. A logic error can be exploited within minutes. Hence the importance of security audits, thorough testing and bug bounty programs before going to production.
Why it matters when hiring
A smart contract developer is not like other back-end developers: every bug is potentially an irreversible financial loss. The good signal: contracts in production, audited, and the ability to explain classic flaws (reentrancy, oracle manipulation) and how they guard against them. Also ask whether they have taken part in an audit, as auditor or audited party.