A matter collineation (sometimes matter symmetry and abbreviated to MC) is a vector field that satisfies the condition,

L X T a b = 0 {\displaystyle {\mathcal {L}}_{X}T_{ab}=0}

where T a b {\displaystyle T_{ab}} are the energy–momentum tensor components. The intimate relation between geometry and physics may be highlighted here, as the vector field X {\displaystyle X} is regarded as preserving certain physical quantities along the flow lines of X {\displaystyle X} , this being true for any two observers. In connection with this, it may be shown that every Killing vector field is a matter collineation (by the Einstein field equations (EFE), with or without cosmological constant). Thus, given a solution of the EFE, a vector field that preserves the metric necessarily preserves the corresponding energy-momentum tensor. When the energy-momentum tensor represents a perfect fluid, every Killing vector field preserves the energy density, pressure and the fluid flow vector field. When the energy-momentum tensor represents an electromagnetic field, a Killing vector field does not necessarily preserve the electric and magnetic fields.