A collection of Unspent Transaction Outputs. This concept is typically used in discussions on optimizing an ever growing index of transaction outputs that are not yet spent. The index is important to efficiently validate newly created transactions. Even if the rate of the new transactions remains constant the time required to locate and verify unspent outputs grows.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc