Glossary
Bytecode
The compiled, EVM-executable representation of a smart contract — what actually lives at the contract's address and what every node executes on call.
A Solidity contract compiles to two streams of bytes. Creation bytecode runs once at deployment; its job is to install the runtime bytecode at the new contract's address. Runtime bytecode is what the EVM executes whenever the contract is later called.
Bytecode is what block explorers verify against the published source: compile the source with the same compiler and settings, strip metadata, and compare. A match proves the deployed contract really is the source it claims to be.