A spacetime diagram (or Minkowski diagram) is a graph of a simplified spacetime which can be used to show consequences of relativity, displaying plotted worldlines. The simplest is a 2-dimensional graph showing only a single dimension of space, i.e., a planar "cut" through 4-dimensional spacetime. Typically, time is represented as the vertical axis. 3-dimensional graphs are also used, showing two space dimensions.
It is common to scale the graph so the path light would take along this dimension shows as a 45-degree angle, i.e., movement at the speed of light. The slope of the line varies inversely with the speed of a small (point-like) object represented by the line, steeper being slower. Lines steeper than the speed-of-light angle can represent objects, i.e.,time-like lines, which could represent the position of the object at each time. Lines less steep than 45 degrees are impossible for a point-like object to follow: that would be exceeding the speed of light: such lines are termed space-like, to indicate in any frame of reference, there will be some space between any two separate points on the line: an object with enough length could sit on the line, interconnecting the points. Variations on the graph, including rotating axes, can be used to demonstrate effect of differing frames of reference.