A dimension is a table in a Star Schema or Snowflake schema of a data warehouse. It is collection of reference information about measurable event. These events are known as facts and stored in a fact table. Dimensions categorize and describe data warehouse facts and measures in way that support meaningful answers to business questions. They form the core of dimensional modeling.
Dimensions provide the "who, what, where, when, why, and how" context surrounding a business process event. Every dimension table has a single primary(surrogate) key. The primary key is embedded as a foreign key in any associated fact table. Dimension tables are usually wide, flat denormalized for analyzing data.