Transaction

💡 The Plain-English Definition

A Bitcoin transaction is the mechanism by which Bitcoin ownership is transferred — a digitally signed instruction that destroys existing chunks of Bitcoin and creates new ones with different ownership conditions. Sending Bitcoin doesn’t move coins; it reassigns them.

🤔 But Why Though?

Bitcoin doesn’t work like a bank account where balances shift when you transfer money. It uses the UTXO model (Unspent Transaction Output — the system of tracking discrete chunks of Bitcoin rather than account balances). Every transaction consumes one or more existing UTXOs as inputs and creates one or more new UTXOs as outputs. The consumed UTXOs are destroyed. The created UTXOs are new, owned by whoever the transaction specifies.

A transaction has three key components. Inputs reference the UTXOs being spent — each input contains a pointer to a specific previous output and a digital signature (cryptographic proof that the owner of the relevant private key authorised this spend). Outputs define new UTXOs — each specifies an amount and a locking condition (typically “the holder of the private key corresponding to this address can spend this”). The fee is implicit — the difference between the total input value and the total output value. This remainder goes to the miner who includes the transaction in a block.

When you “send 0.01 BTC to Alice,” your wallet finds UTXOs you own totalling at least 0.01 BTC, constructs a transaction spending them as inputs, creates one output of 0.01 BTC locked to Alice’s address, creates another output returning the change to your wallet (if your inputs exceeded 0.01 BTC), signs the inputs with your private keys, and broadcasts the transaction to the network. Miners pick it up from the mempool (Bitcoin’s waiting room for unconfirmed transactions), include it in a block, and once confirmed, Alice’s new UTXO exists — locked to her key alone.

🌍 The Real-World Analogy

Think of Bitcoin transactions like paying with physical banknotes — except the banknotes are destroyed and reissued with each payment. You have a £50 note (a UTXO). You want to pay Alice £30. You hand the £50 note to a trusted minting authority (the miners), who destroy it and instantly create a new £30 note for Alice and a new £20 note for you (change). The original note no longer exists. Two new notes exist in its place, each with clear ownership. Nobody “moved” money — the old form was destroyed and the new forms created.