Smart Contract
A program stored on a blockchain that runs exactly as written whenever someone sends it a transaction.
A contract lives at its own address with its own code and storage, and every node executes it identically so all of them agree on the result. Contracts implement tokens, exchanges, lending markets, bridges, and governance, and once deployed the code cannot be edited unless the developers built in an upgrade mechanism, which reintroduces an administrator who can change behavior. Because execution is automatic, a bug is executed just as faithfully as intended logic, and there is no support desk that can reverse the outcome; audits and formal verification reduce this risk without removing it. Readers meet the term whenever an application asks a wallet to approve an interaction.
In practice
A token contract implementing the ERC-20 interface holds a mapping of balances, and transferring the token means calling that contract rather than moving anything at the protocol level.
The common misunderstanding
Smart does not mean intelligent or legally binding; the code enforces its own rules and cannot tell whether the result was fair or intended.