From the documentary "The Elegant Universe". http://www.pbs.org/wgbh/nova/elegant/
Space-Time And The Speed Of Light - The concept of spacetime combines space and time to a single abstract "space", for which a unified coordinate system is chosen. Typically three spatial dimensions (length, width, height), and one temporal dimension (time) are required. Dimensions are independent components of a coordinate grid needed to locate a point in a certain defined "space"
