Smart Contract
Self-executing code deployed on a blockchain that runs exactly as written.
A smart contract is a program stored on-chain. Once deployed it runs automatically when its conditions are met, and in most cases nobody can stop or alter it, including its author.
That immutability is the point and also the danger. Every DeFi exchange, lending market and NFT collection is built on them, and a single logic flaw can drain the contract permanently, which is why audits matter.