The word Data Warehouse(DWH) first came from Bill Inmon who is recognized by many as the father of the data warehouse. According to him-
A data warehouse is a subject oriented, nonvolatile, integrated, time variant collection of data in support of management decisions.
A data warehouse can be utilized to analyze data for a particular subject area’s data. For example, customer can be a particular subject. Using this warehouse, you can answer questions like – How many new customer added in last month.
Non-volatile means that once data is entered into the data warehouse, it should not change. This will ensure to maintain the historical data. It is obvious as the whole purpose of data warehouse is to analyze the trends.
A data warehouse is built by the data coming from different types of source. That means source may be any kind of operational system and method of identifying a product may be different but in data warehouse all the data will be integrated and only one method will be there to identify a single product.
Time variant means historical data will be maintained in data warehouse. For example, in conventional operational system only current data used to maintain wherein from data warehouse you can get retrieve data older than one year or even more.