dimensions — Mathematics. a property of space; extension in a given direction: A straight line has one dimension, a parallelogram has two dimensions, and a parallelepiped has three dimensions. the generalization of this property to spaces with curvilinear extension, as the surface of a sphere. the generalization of this property to vector spaces and to Hilbert space. the generalization of this property to fractals, which can have dimensions that are noninteger real numbers. extension in time: Space-time has three dimensions of space and one of time.
