Input (Transaction Input)
A reference to a specific UTXO being spent, including a signature unlocking those funds.
A transaction input is a pointer to a UTXO being spent, plus the proof that the spender is allowed to spend it.
Each input references the UTXO by its source txid (the transaction that created it) and output index (which output of that transaction). It also carries unlocking data - a digital signature for legacy transactions, or witness data for SegWit transactions - that satisfies the UTXO's locking script.
A single transaction can have many inputs. Spending three UTXOs in one transaction means three inputs, each needing its own valid signature. If any one input fails to validate, the entire transaction is rejected by every node - inputs are atomic with the transaction containing them.
Once a transaction confirms, every input it referenced is permanently consumed. Trying to reuse an already-spent UTXO is what makes a double spend attempt; the network rejects it.
Key takeaways
- Identifies which UTXO is being spent
- Must contain valid signature/witness data
- Multiple inputs can fund one transaction