In mathematics and physics, a dimension is a measure of the size or distance of an object or space in one direction. It is the minimum number of coordinates needed to specify any point within a mathematical space. In other words, the dimension is the number of independent parameters or coordinates that are needed to define the position of a point that is constrained to be on the object. For example, a point has zero dimensions, a line has one dimension, and a plane has two dimensions.

Dimensions are an intrinsic property of an object, meaning that they are independent of the dimension of the space in which the object is or can be embedded. For instance, a curve such as a circle is of dimension one because the position of a point on a curve is determined by its signed distance along the curve to a fixed point on the curve, which is independent of the fact that a curve cannot be embedded in a Euclidean space of dimension lower than two, unless it is a line.

In data warehousing, a dimension is a collection of reference information that supports a measurable event, such as a customer transaction. Dimensions provide the details necessary to understand and analyze a set of related facts. For example, a data warehouse might include a set of facts about a companys product sales, and each sale is a fact that reflects what product was sold, when it was sold, who bought the product, how much it cost, and other relevant information. Dimensions support these facts by providing the background information needed to understand each sale.