Transaction
A signed statement that moves value from one address to another, recorded permanently on the ledger.
In simple terms
A transaction names the previous outputs it spends, the new outputs it creates, and carries a signature proving the owner authorised it.
Analogy
Tearing off numbered coupons from your cash and handing them over — the cashier checks your signature, then hands back your change coupons.
Going deeper
Bitcoin's UTXO model spends discrete outputs (parallel-friendly, private by default); Ethereum's account model debits and balances like a bank ledger (friendlier for smart contracts). Either way, nodes verify signatures and no-double-spends before relaying.
Common mistake
Confirmed transactions are irreversible. There is no chargeback — verify addresses character by character before sending.