The Einstein tensor is a mathematical object used in the field of general relativity, formulated by Albert Einstein. It describes the curvature of spacetime caused by mass and energy. The tensor is denoted as G_\mu\nu and relates to the distribution of matter through the Einstein field equations. In these equations, the Einstein tensor is equated to the stress-energy tensor, which represents the energy density and momentum of matter. This relationship shows how matter influences the geometry of spacetime, leading to the gravitational effects we observe in the universe.