Catalyst
A substance that increases the rate of a reaction by providing an alternative pathway of lower activation energy, without being consumed.
Unit
Dimensionless
In depth
A catalyst is regenerated in a later step, so a tiny amount can transform vast quantities of reactant. It speeds forward and reverse reactions equally, so it accelerates the approach to equilibrium without shifting its position or changing ΔH or ΔG.
Examples in the real world
Iron in the Haber process, vanadium pentoxide in the Contact process, and platinum in a catalytic converter.