An input (Input) is an element of the transaction structure in the Bitcoin blockchain and similar networks that determines where the funds being transferred come from. In essence, an input references the output of one of the previous transactions where these coins were received. In this way, each transaction relies on previously confirmed receipts, forming a continuous chain of ownership.

Most often, an input contains a reference to an unspent output and proof of the right to dispose of it — the sender's digital signature. The exception is a special transaction (coinbase), which creates new coins as a reward for a mined block: it has no ordinary input referencing a previous transaction, since the coins "appear" in the network for the first time.

The role of the input in a transaction

• It indicates the source of funds — previously received coins.
• It contains a signature confirming the right to dispose of the coins.
• Together with outputs, it forms the complete structure of the transfer.

Understanding how inputs and outputs work helps to grasp the UTXO accounting model on which Bitcoin and a number of other blockchains are built. This model ensures the transparency and verifiability of every transfer in the network.