A "reaction coordinate" is envisioned as a continuous transformation from reactant-like to product-like geometry along a minimum-energy pathway, passing through the high-energy transition state geometry. The intrinsic reaction coordinate (IRC) is a particular choice that corresponds to infinitely slow passage along a steepest-descent trajectory from the transition state towards the connecting reactant or product equilibrium minima, with complete equilibration (optimization) of all remaining geometrical degrees of freedom.