Complex sentence: The stress-energy tensor (sometimes stress-energy-momentum tensor) is a tensor quantity in physics that describes the density and flux of energy and momentum in spacetime, generalizing the stress tensor of Newtonian physics.
Split into simpler sentences: The stress-energy tensor (sometimes stress-energy-momentum tensor) is a tensor quantity in physics. It describes the density and flux of energy and momentum in space, generalizing the stress tensor of Newtonian physics.