Definitions of cloud computing are easy to find, but a single, authoritative definition is hard to come by. Perhaps the best work in this area was done by Böhm, et al.
By compiling characteristics of 17 different scholarly and industrial definitions, the authors identified five primary characteristics of cloud computing allowing a definition such as: "Cloud computing is a service that delivers scalable hardware and/or software solutions via the Internet or other network on a pay-per-usage basis." (Emphasis indicates essential definition elements). Source: The Evolution of Cloud Computing