Invariant interval
In physics, the invariant interval is the measure of separation between two arbitrarily close events in the spacetime of general or special theory of relativity. It is invariant under the coordinate transformations from the covariance group of the theory. That is, in special relativity it is invariant under Lorentz transformations; in general relativity it is invariant under arbitrary invertible differentiable coordinate transformations.
The invariant interval is usually denoted as 
and is given in terms of the metric tensor 
as
where 
is the coordinate differential between the two events.