A Spacetime Diagram (or Minkowski Diagram) is a graph of a simplified Spacetime which can be used to demonstrate consequences of Relativity. It is a 2-dimensional graph showing only a single dimension of space, i.e., a planar "cut" through 4-dimensional spacetime. (A variant on such a diagram is a 3-dimensional graph that shows two space dimensions.) Typically, time is represented as the vertical axis.
Typically the graph is scaled so the path light would take along this dimension shows as a 45-degree angle. Lines steeper but not vertical can represent objects, i.e., Time-Like, lines, and lines less steep are impossible to traverse (Space-Like). Variations on the graph, including rotating axes, can be used to demonstrate effect of differing Frames Of Reference.