Catalyst
Definition
A substance that speeds up a reaction without being used up: it offers an easier path with lower activation energy, then emerges unchanged.
Technical definition
A catalyst increases reaction rate by providing an alternative mechanism with lower activation energy. It is not consumed in the overall reaction, does not change the Gibbs energy or the equilibrium position, and works by stabilizing the transition state.
Example
The catalytic converter in a car uses platinum to convert toxic CO into CO2. The platinum is not used up, and the exhaust gases' final equilibrium is unchanged.
Common confusion
Catalysts cannot make impossible reactions happen. They cannot change thermodynamics, so a reaction with positive deltaG stays nonspontaneous no matter the catalyst.