A data warehouse is a central repository for all or most of the data that a business collects through its various systems. Most data warehouses are used for data minining and decision support systems (DSS). It is generally kept seperate from the operational systems, that is, systems that handle the day-to-day processing of information for various business-related tasks or applications.

Sources:
http://www.whatis.com/dataware.htm
http://www.cait.wustl.edu/papers/prism/vol1_no1/