Let be the matrix of a Lorentz transformation acting on Minkowski spacetime.
Let's take a look at its diagonal component (in our convention it's ) that transforms the time component of . Start from the well known identity which defines Lorentz transformations:
Now this gives us a way to classify Lorentz transformations:
If , the transformation does not preserve the direction of time and is called non-orthochronous
If , the transformation preserves the direction of time and is called orthochronous