A part of a transaction that contains a reference to a previous transaction’s output and a script that can prove ownership of an output. The script usually contains a signature and is thus called scriptSig. Inputs spend previous outputs completely. Therefore if one needs to pay only a portion or some previous output, the transaction should include extra change output that sends the remaining portion back to its owner on the same or different address. Coinbase transactions contain only one input with a zeroed reference to a previous transaction and arbitrary in place of script.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc