Space-time
The set of 3 coordinates to describe the position of an object is relative to the object being described.
An event is something that happens at a particular point in space and at a particular time. So one can specify it by four numbers or coordinates. Again, the choice of coordinates is arbitrary; one can use any three well-defined spatial coordinates and any measure of time. In relativity, there is no real difference between the space and time coordinates, just as there is no real difference between any two space coordinates. One can also choose a new set of coordinates in which some are a combination of the old coordinates.
It is often helpful to think of the four coordinates of an event as specifying its position in a four-dimensional space called space-time.