The big important equation of

General Relativity:

R_{αβ} - ½g_{αβ}R^{γ}_{γ}=8πT_{αβ}

where

...and you may or may not want to include the

cosmological constant in there.

Referred to as equation*s* 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, G_{αβ}), and on the right is a tensorial characterisation of a mass-energy fluid.