Quote by Matterwave
Depending on your metric, it's either equal to the proper time, or equal to the squareroot of the negative of the square of the proper time (i.e. [itex]ds=\sqrt{d\tau^2}[/itex]).

Then if the spacetime interval is timelike it is numerically equal to the proper time.
If the spacetime interval is spacelike then it is numerically equal to the proper distance ?