From the SAP Terminology Guide:
"A software architecture that supports the design, development, identification, and consumption of standardized services across the enterprise, thereby improving reusability of software components and creating agility in responding to change." (Source: industry term.)
Service-oriented architecture, or SOA is a style of software architecture that uses services as the fundamental building blocks.
Service Oriented Architecture consists of three types of components:
Service Providers - an independent entity which provides one or more services to the service consumers and publishes its contract in the registry.
Service Consumers - a software application or the end user which utilizes the service of the service provider.
Service Broker - can serve as a mediator for communication between service providers and service consumers (thereby eliminating rigid point-2-point or peer-2-peer communication). Also maintains or hosts a services registry where service providers can publish their service contracts and consumers can discover potential servicess that can be used.