Feature on a potential-energy surface whereby a minimum-energy reaction path (MERP) emanating from a saddle point (corresponding to a transition structure) splits in two and leads to alternative products without intervening minima or secondary barriers to overcome. A bifurcation arises when the curvature of the surface in a direction perpendicular to the MERP becomes zero and then negative; it implies the existence of a lower-energy transition structure with a transition vector orthogonal to the original MERP.