Basis is a Business Application Aoftware Integrated Solution. Simply, Basis is the administration of the SAP system.
A set of middleware programs and tools that provide the underlying base that enable applications to be interoperable across operating systems. SAP Basis includes a RDBMS, GUI, and client server architecture. Beyond the interface aspect of Basis, it also includes such components as a data dictionary as well as user and system administration.
It's a piece of middleware which links the application with the database and the operating system. Basis is most commonly associated with the GUI interface to the SAP, and the Basis administrator is an SAP, professional who is responsible for configuring the SAP environment, including the GUI screens and SAP application servers.