Smart contract
Code that lives on a blockchain and is executed by every node when someone calls it. The term was coined by Nick Szabo in 1994; Ethereum made it practical in 2015.
Usually written in Solidity, compiled to EVM bytecode, and immutable once deployed — hence upgrade patterns built on proxy contracts. "Code is law" held until the 2016 DAO hack, when 3.6 million ETH was drained through a reentrancy bug and the Ethereum community hard-forked the event away (what remained is Ethereum Classic). Common vulnerability classes: reentrancy, integer overflow (less so since Solidity 0.8), oracle manipulation, front-running. Every line costs gas, so optimization and safety pull in opposite directions.