A measure of spatial extent, especially width, height, or length. (See the full wiki for additional meanings.)
Disambiguation of uses in GIS:
(Noun) The numeric coordinates needed to locate a point within a space or a map (two for points on a sphere or plane, three for points in space, four for points in space-time, for instance).
(Noun) The number of numeric coordinates needed to distinguish points in a feature (zero for a point; one for a polyline, curve, or arc; two for a region; three for a solid).
(Noun) The display of "specific lengths or distances on a map" (http://www.esri.com/news/arcuser/0703/files/dimensions.pdf).
(Verb) To cut or shape something to a specific size or measurements (http://www.thefreedictionary.com/dimension inter alia).
(Noun) A fundamental measure of a physical quantity, such as length, duration, or mass.