Winding order
The order — clockwise or counter-clockwise — in which a triangle's vertices are listed, which decides the side the renderer treats as the front.
A triangle has no inherent front or back; that is set by the order its three vertices are listed in. If they appear to run counter-clockwise on screen, the GPU reads the face as pointing toward us, clockwise as facing away — which is which is a configurable convention. All of back-face culling rests on this: faces with the wrong winding are thrown out before they are drawn.
The decision happens after the triangles are projected into clip space, where the on-screen vertex order can be read off. Let a model mix its ordering and you get invisible triangles — a classic bug when meshes are imported or mirrored, since mirroring flips the winding.