  In the single fluid, nonrelativistic, ideal-Magnetohydrodynamic (MHD) plasma
description magnetic field lines play a fundamental role by defining
dynamically preserved "magnetic connections" between plasma elements. Here we
show how the concept of magnetic connection needs to be generalized in the case
of a relativistic MHD description where we require covariance under arbitrary
Lorentz transformations. This is performed by defining 2-D {\it magnetic
connection hypersurfaces} in the 4-D Minkowski space. This generalization
accounts for the loss of simultaneity between spatially separated events in
different frames and is expected to provide a powerful insight into the 4-D
geometry of electromagnetic fields when ${\bf E} \cdot {\bf B} = 0$.
