yes, and what is this constant called?

The speed of light

No, it is not the speed of light

It is the : space_time interval

Suppose you have two events and one observer

The distance beetwen the two events is : AB
The time difference beetwen the events is T
In this period the light moves : AC = C*T

An other observer can measure different values of AB and T an consequently of AC
but the value :

AC^2 - AB^2

Is the same for any observer