origin (of graph)
This is the term used for the starting place on a graph. We usually use the letter O to label the origin.
On a two-dimensional graph, the origin is where the x-axis and the y-axis cross. These are the coordinates (0, 0).
On a three-dimensional graph, the origin is where the x-axis, the y-axis and the z-axis cross. These are the coordinates (0, 0, 0).