From Enterprise SOA by Krafzig et al:
...a software architecture that is based on the key concepts of an application frontend, service, service repository, and service bus. A service consists of a contract, one or more interfaces, and an implementation.
From Understanding SOA with Web Services by Newcomer and Lomow:
...a style of design that guides all aspects of creating and using business services throughout the lifecycle (from conception to retirement). ...a way to define and provision an IT infrastructure to allow different applications to exchange data and participate in business processes, regardless of the operating systems or programming languages underlying those applications... an approach to building IT system in which the business services [...] are the key organizing principle using to align IT systems...