The big important equation of General Relativity:
    Rαβ - ½gαβRγγ=8πTαβ
where ...and you may or may not want to include the cosmological constant in there.

Referred to as equations in the plural, as it gives an equation for each assignment α,β=0,1,2,3 however, it is a valid tensor equation (in the singular), which is how you really should think of it (not them).
Put simply, what you have on the left hand side is and expression for the curvature of space-time (sometimes called the Einstein tensor, Gαβ), and on the right is a tensorial characterisation of a mass-energy fluid.

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