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In common usage, a dimension (Latin, "measured out") is a parameter or measurement required to define the characteristics of an object—i.e. length, width, and height or size and shape. In mathematics, dimensions are the parameters required to describe the position and relevant characteristics of any object within a conceptual space —where the dimensions of a space are the total number of different parameters used for all possible objects considered in the model. Generalizations of this concept are possible and different fields of study will define their spaces by their own relevant dimensions, and use these spaces as frameworks upon which all other study (in that area) is based. In specialized contexts, units of measurement may sometimes be "dimensions"—meters or feet in geographical space models, or cost and price in models of a local economy.
For example, locating a point on a plane (e.g. a city on a map of the Earth) requires two parameters — latitude and longitude. The corresponding space has therefore two dimensions, its dimension is two, and this space is said to be 2-dimensional (2D). Locating the exact position of an aircraft in flight (relative to the Earth) requires another dimension (altitude), hence the position of the aircraft can be rendered in a three-dimensional space (3D). Adding the three Euler angles, for a total 6 dimensions, allows the current degrees of freedom —orientation and trajectory —of the aircraft to be known.
Time can be added as a 3rd or 4th dimension (to a 2D or 3D space, respectively). Then the aircraft's estimated "speed" may be calculated from a comparison between the times associated with any two positions. For common uses, simply using "speed" (as a dimension) is a useful way of condensing (or translating) the more abstract time dimension, even if "speed" is not a dimension, but rather a calculation based on two dimensions.
Theoretical physics often experiments with dimensions - adding more, or changing their properties - in order to describe unusual conceptual models of space, in order to help better describe concept of quantum mechanics —ie. the 'physics beneath the visible physical world.' This concept has been borrowed in science fiction as a metaphorical device, where an "alternate dimension" (ie. 'alternate universe' or 'plane of existence') describes Extraterrestrial places, species, and cultures which function in various different and unusual ways from human culture.
The physical dimensions are the parameters required to answer the question where and when some event happened or will happen; for instance: When did Napoleon die? — On the 5 May 1821 at Saint Helena (15°56' S 5°42' W). They play a fundamental role in our perception of the world around us. According to Immanuel Kant, we actually do not perceive them but they form the frame in which we perceive events; they form the a priori background in which events are perceived.
The complete, up-to-date and editable article about Dimension can be found at Wikipedia: Dimension