Glossary
Transaction
A signed message from an account that, when included in a block, mutates blockchain state — moving funds, calling a contract, or both.
A transaction is the only way ordinary state changes happen on a blockchain. It is a message signed by the sender's private key, authorizing some action (transfer, contract call, contract deployment) and paying a fee for the work the network does to include it.
Once mined or attested into a block, the transaction's effects become part of the canonical state. Replays are prevented by a nonce (an incrementing counter per account) so the same transaction cannot be applied twice.