From the SAP Terminology Guide:
"A composition platform that allows organizations to compose new business solutions quickly, while obtaining more business value from existing IT investments. SAP NetWeaver is the foundation for enterprise SOA, enabling organizations to evolve their current IT landscape into a strategic environment that drives business change.
SAP NetWeaver is an open technology platform that unifies technology components in a single platform, reducing the need for custom integration and ensuring that mission-critical business processes are reliable, secure, and scalable.
SAP NetWeaver powers the SAP Business Suite family of business applications, SAP xAPPs, composite applications, and partner solutions. It provides the best way to integrate all systems running SAP or non-SAP software.
Initially launched as an integration and application platform, SAP NetWeaver has evolved to become a composition platform, allowing model-based development to enhance open enterprise services that are delivered by SAP Business Suite applications. In 2006, SAP NetWeaver will be enhanced to extend its use as a business process platform." (Source: SAP)