Time for me to go into math mode. Ignoring string theory for now because I don't want to deal with it, position in the universe is governed by the function f(x,y,z,t) where x,y,z,and t are unit vectors that form an orthonormal basis. That is x,y,z,and t are all perpendicular to each other. x,y, and z are defined on all real numbers, but t is parameterized such that t>0 and is unilateral. x,y,and z are called the spatial dimensions and t is dimension known as time.