  The famous van Benthem theorem states that modal logic corresponds exactly to
the fragment of first-order logic that is invariant under bisimulation. In this
article we prove an exact analogue of this theorem in the framework of modal
dependence logic MDL and team semantics. We show that modal team logic MTL,
extending MDL by classical negation, captures exactly the FO-definable
bisimulation invariant properties of Kripke structures and teams. We also
compare the expressive power of MTL to most of the variants and extensions of
MDL recently studied in the area.
