A dimension is a set of structured data labels that help describe an event (also called a fact), facilitating data searches and creating business intelligence. For example, if the event in question were the sale of an automobile at a car dealership, the dimension would be labels describing the car (categories and subcategories), all of which would be contained in a dimension table.
Dimensions help categorize products, customers, time, geography, etc. to facilitate queries and to illuminate the circumstances surrounding an event, in order to create knowledge that will lead to better business decisions in the future.
http://en.wikipedia.org/wiki/Dimension_%28data_warehouse%29 http://searchdatamanagement.techtarget.com/definition/dimension http://docs.oracle.com/cd/B19306_01/server.102/b14223/dimen.htm