The starting point.

On a number line it is 0

On a two-dimensional graph it is where the X axis and Y axis cross, marked (0, 0) on the graph here:

In three dimensions it is the point (0, 0, 0)

Often written as the letter O

On a number line it is 0

On a two-dimensional graph it is where the X axis and Y axis cross, marked (0, 0) on the graph here:

In three dimensions it is the point (0, 0, 0)

Often written as the letter O