It is multiplied by the m
It is multiplied by the metric, and not the stress-energy, tensor in the field equations.  Observations of Type 1a supernovae have given support to the existence of this quantity, equivalent to the energy density of the vacuum.  It's also possible to construct a steady-state model of the universe using, for ten points, what constant which Einstein dubbed his "greatest mistake?"
It is multiplied by the metric, and not the stress-energy, tensor in the field equations.  
cosmological constant or cosmological term